Emulsion for robust sensing

ABSTRACT

The present invention provides an optical fluorescence based sensor for measuring the concentration of a gas (e.g., CO 2  or ammonia) in a medium such as blood which has superior dry web sensor performance, enhanced sensor consistency for transparent sensor calibration, improved autoclave stability and rapid rehydration of the sensor. In a preferred embodiment, the sensors of the present invention comprise microcompartments of an aqueous phase having a pH sensitive indicator component and a nonionic amphipathic surfactant within a hydrophobic barrier phase comprising a plurality of dispersed hydrophobic particles.

This is a continuation of application Ser. No. 08/562,036 filed Nov. 22,1995, now U.S. Pat. No. 5,714,122.

FIELD

The present invention relates generally to sensors for measuring theconcentration of an analyte of interest. In a preferred embodiment, thepresent invention relates to sensors for monitoring blood gasconcentrations (e.g., carbon dioxide). The present invention alsorelates to methods of making stable and reproducible water in oilemulsions comprised of a dispersed aqueous phase and a hydrophobiccontinuous phase.

BACKGROUND

It is sometimes necessary or desirable for a physician to determine theconcentration of certain gases, e.g., oxygen and carbon dioxide, inblood. This can be accomplished utilizing an optical sensor whichcontains an optical indicator responsive to the component or analyte ofinterest. The optical sensor is exposed to the blood, and excitationlight is provided to the sensor so that the optical indicator canprovide an optical signal indicative of a characteristic of the analyteof interest. For example, the optical indicator may fluoresce andprovide a fluorescent optical signal or it may function on theprinciples of light absorbance.

The use of optical fibers has been suggested as part of such sensorsystems. The optical indicator is placed at the end of an optical fiberwhich is placed in contact with the medium to be analyzed. This approachhas many advantages, particularly when it is desired to determine aconcentration of analyte in a medium inside a patient's body. Theoptical fiber/indicator combination can be made sufficiently small insize to easily enter and remain in the cardiovascular system of thepatient.

Optical fluorescence CO₂ sensors commonly utilize an indirect method ofsensing based on the hydration of CO₂ to form carbonic acid within anoptionally buffered aqueous compartment containing a pH sensitive dye.The aqueous compartment is encapsulated in a barrier material which isimpermeable to hydrogen ions but permeable to CO₂. An opticallyinterrogated pH change in the internal aqueous compartment can then berelated to the partial pressure of CO₂ in the monitored sample. Ionicisolation of the internal aqueous phase may be achieved by directlydispersing aqueous droplets throughout the isolating matrix.Alternatively, the aqueous phase may be sorbed into porous particleswhich are then dispersed throughout the isolating matrix. The isolationmatrix or “barrier” is typically a crosslinked silicone polymer.

Unfortunately, prior attempts to provide stable and reproducibleemulsions of an aqueous phase dispersed in a polymeric precursor haveyielded poor results. In some cases, the emulsions exhibited unexplainedlot-to-lot variability that frustrate attempts to perform quantitativeexperiments correlating sensor performance to the particular sensorformulation. Variability within lots has also been observed. Thisvariability frustrates attempts to uniformly produce sensors, e.g., bycoating a sheet of sensor precursor and converting the sheet intoindividual sensor elements. In other cases, the emulsoids formed fromthe emulsions are adversely affected by heat (e.g., during autoclaving)and the sensor's performance is thereby compromised. Also unfortunately,prior attempts to make sensors that respond to CO₂ in the “dry” state,i.e., not in contact with liquid water, have yielded poor results.Traditional two-phase sensors dehydrate when stored in ambientconditions and lose intensity. Even when intensity is maintained in thedry state, the sensor may not respond to CO₂.

It would be desirable to provide a stable and reproducible sensor whichhas a fast response time and which is easily manufactured. It would alsobe desirable to provide a CO₂ sensor that provides a stable andeffective signal that does not require that it be held in a condition ofequilibrium with liquid water or saturated water vapor.

SUMMARY

We have discovered a stable and reproducible sensor. This sensor employsthe preparation of stable water in oil emulsions comprised of adispersed aqueous phase and a hydrophobic continuous phase. Morespecifically, this invention provides a novel method of preparingaggregation and coalescence resistant emulsions for use as blood gassensor compositions, and further discloses novel emulsion compositionssuitable for use in consistently and uniformly manufacturing precisioncoated blood gas sensors.

In one embodiment, the invention provides a gas sensing composition,comprising a dispersed first phase comprising droplets which aresubstantially smaller in at least one dimension than the thickness ofthe sensing composition and a hydrophobic second phase which ispermeable to the analyte and impermeable to ionized hydrogen. The firstphase contains at least one substantially water soluble emulsificationenhancement agent and at least one water soluble indicator componenteffective to provide a signal in response to the concentration of a gasin a medium to which the sensing composition is exposed. The secondphase contains at least one substantially water insoluble emulsificationenhancement agent. Preferably the water soluble emulsificationenhancement agent comprises a nonionic, amphipathic copolymer containingboth hydrophilic and hydrophobic moieties, and the water insolubleemulsification enhancement agent comprises a plurality of dispersedhydrophobic particles.

The improved sensor compositions exhibit superior dry web sensorperformance, enhanced sensor consistency for transparent sensorcalibration, improved autoclave stability and rapid rehydration of thesensor.

The improved sensors may be used to sense the concentration of ananalyte of interest in a medium. More particularly, the improved sensorsmay be used to sense carbon dioxide in blood. The invention also relatesto sensor apparatus or systems and methods for sensing the concentrationof other analytes of interest in industrial settings and environments(e.g., ammonia, CO₂, SO₂, or NO₂).

Precision coated blood gas sensors are provided which exhibit improvedcoating uniformity and consistent sensor performance from lot to lot,thereby increasing the yield of usable sensors and permittingtransparent calibration of the sensors independent of coating time orlot. By transparent calibration we mean the designation of a sensor weblot by the manufacturer according to its calibration parameters (slopeand/or intercept) achieved by a web sampling plan. Transparentcalibration allows the user to use a constant set of calibrationparameters and significantly reduces the calibration time required bythe user. It is dependent on consistent performance across and downsensor webs.

In another embodiment, the present invention provides a “dry” gassensing composition, comprising a dispersed first phase containing ahumectant (preferably glycerol) and at least one soluble indicatorcomponent; and a hydrophobic second phase which is permeable to theanalyte and impermeable to ionized hydrogen. The first phase optionallycontains a water soluble emulsification enhancement agent as describedabove. The second phase optionally, and preferably, contains at leastone substantially water insoluble emulsification enhancement agent. Thesensing composition of this embodiment provides an effective signal inresponse to the gas without the necessity that it be held in a conditionof equilibrium with liquid water or water vapor prior to use.

In yet another embodiment, the present invention provides a precisioncoated blood gas sensor which exhibits improved autoclave stability, andpreferably also exhibits improved dry web shelf stability. Mostpreferred blood gas sensors also exhibit rapid rehydration rates.

RELATED APPLICATIONS

Of related interest are copending U.S. patent application Ser. No.08/375,304, now U.S. Pat. No. 5,607,645 “Sensor with Improved DriftStability”; Ser. No. 08/159,799, now U.S. Pat. No. 5,508,509 “SensingElements and Methods for Making Same”; Ser. No. 08/136,967, now U.S.Pat. No. 5,462,879 “Emission Quenching Sensors”; and Ser. No.08/137,289, now U.S. Pat. No. 5,409,666 “Sensors and Methods forSensing,” the disclosures of which are herein incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood when taken in conjunction withthe drawings wherein:

FIG. 1 is an elevational view in section of a droplet of materialutilized in the preparation of a gas sensor of the invention;

FIGS. 2a, 2 b, 2 c, and 2 d are views in section of a gas sensor of theinvention;

FIG. 3 is a partial view in section of a gas sensor of the presentinvention which comprises a flow through cassette comprising a sensingelement; and

FIGS. 4 and 5 are two partial views in section of a gas sensor of thepresent invention which comprise a flow through cassette comprising apreformed laminate sheet sensing element.

This invention utilizes certain principles and/or concepts as are setforth in the claims appended to this specification. Those skilled in thegas sensing arts to which this invention pertains will realize thatthese principles and/or concepts are capable of being illustrated in avariety of embodiments which may differ from the exact embodimentsutilized for illustrative purposes in this specification. For thesereasons, the invention described in this specification is not to beconstrued as being limited to only the illustrative embodiments but isonly to be construed in view of the appended claims.

DEFINITIONS

As used herein, the terms “aqueous first phase” or “aqueous phase” referto the hydrophilic phase or phases of a multiphase sensor whichcomprises an indicator component (“dye”) and which more preferably, butnot necessarily, further comprises water.

As used herein, the terms “hydrophobic second phase” or “hydrophobicphase” refer to that phase of a multiphase sensor which separates anaqueous phase (comprising an indicator component) from the mediumcontaining the analyte of interest. As used herein, the terms “polymericphase” or “silicone phase” refer to a hydrophobic second phase whichcomprises a polymer material or silicone material, respectively.

As used herein, the term “medium” refers to the solid, liquid, orgaseous environment to which the sensor is exposed. For example, sensorsare often placed into a medium of blood so that the blood gasses (e.g.,CO₂ level) may be measured. Sensors may also be placed in a gas (or CO₂)equilibrated liquid medium (e.g., to calibrate a sensor).

As used herein, the term “emulsion” refers to a uniform multi-phasesystem of two or more liquids and includes multi-phase suspensions anddispersions of droplets of one liquid, comprising the dispersed orinternal phase, in a second substantially insoluble and immiscibleliquid, comprising the continuous or external phase. The use of the termemulsion is not limited to thermodynamically stable mixtures or mixturescontaining emulsifiers. As used herein, the term “stable emulsion”refers to emulsions which remain substantially uniform (macroscopically)for a long enough period of time to allow the emulsion to be formed intothe desired configuration, e.g., a period of at least several hours. Asused herein, the term “thermodynamically stable emulsion” refers toemulsions which remain substantially uniform (macroscopically) even whenheated and then cooled.

As used herein, the term “emulsoid” refers to a multi-phase systemcomprising micro-compartments of a dispersed phase in a second solidphase (e.g., a cross-linked polymer phase).

As used herein, the terms “emulsification enhancement agent” (EEA) or“emulsifier” refer to a substance which acts alone or together withanother emulsification enhancement agent or emulsifier to facilitate theformation and promote the stability of an emulsion.

As used herein, the terms “surface active agent” or “surfactant” referto chemical substances which are strongly adsorbed at an interface,thereby causing a substantial lowering of the surface (or interfacial)tension.

As used herein, the term “amphipathic compound” refers to a molecularstructure characteristic of surface active agents which form micelles.Amphipathic molecules consist of a chemical structural group (thelyophobic group) having very little affinity for the solvent (hence lowsolubility in that solvent), combined with a chemical structural group(the lyophilic group) having very high affinity for the solvent (hencehigh solubility in that solvent). When the solvent is water orpredominantly aqueous, the lyophobic group is described as hydrophobicand the lyophilic group is described as hydrophilic.

As used herein, the term “cloud point” refers to the maximum temperatureat which micelles are stable with respect to aggregation. Attemperatures above the cloud point, micelles will either coalesce intoan insoluble bulk surfactant phase, or aggregate and settle into aconcentrated sediment of large micellar aggregates. The cloud point ismarked by a change in physical appearance from a visually transparentsolution to a visually turbid, multiphase dispersion.

As used herein, the term “hydrophile-lipophile balance” (HLB) refers toan empirical scale ranging from 0 to 40 used for representing theamphipathic nature of a surface active agent. Low values correspond tomore hydrophobic surfactants, while high values correspond to morehydrophilic surfactants. For nonionic surfactants which do not contain apolyoxypropylene, polyoxybutylene or polydimethylsiloxane group, the HLBis determined using the methods of Griffin [Griffin, W. C., J. Soc.Cosmetic Chemists, 1, 311 (1949), Griffin, W. C., J. Soc. CosmeticChemists, 5, 249 (1954).]. For nonionic surfactants containing thepolyoxypropylene or polyoxybutylene groups, the HLB is determined bymeasuring gas-liquid chromatography (GLC) relative retention ratiosusing the method of Becher and Birkmeier (Becher, P. and Birkmeier, R.L., J. Am. Oil Chemists Soc., 41, 169 (1964).]. For nonionic surfactantscontaining the polydimethylsiloxane group in conjunction withpolyoxyethylene, polyoxypropylene or polyoxybutylene, the HLB isdetermined from cloud point measurements using the method of Griffin[Griffin, W. C., Off. Dig. Fed. Paint and Varnish Production Clubs, 28,446 (1956); and Schott, H., J. Pharm. Science, 58, 1443 (1969) (citingGriffin)]. The aforementioned references are herein incorporated byreference.

As used herein, the term “hydrophilic particle” refers to afinely-divided, substantially water insoluble solid, with a mean volumeaverage diameter less than 5 microns, the surface of which may betreated with an organic compound and which exhibits a high affinity forwater, as reflected by a solid/liquid contact angle (measured throughthe aqueous phase) of less than 90 degrees.

As used herein, the term “hydrophobic particle” refers to afinely-divided, substantially water insoluble solid, with a mean volumeaverage diameter less than 5 microns, the surface of which may betreated with an organic compound and which exhibits a low affinity forwater, as reflected by a solid/liquid contact angle (measured throughthe aqueous phase) of greater than 90 degrees.

As used herein, the term “micelle” refers to an organized cluster ofamphipathic surfactant molecules dissolved in a solvent, in which thelyophobic groups of the individual molecules are oriented towards theinterior of the cluster (i.e., away from the solvent), and the lyophilicgroups of the individual molecules are oriented towards the exterior ofthe cluster (i.e., towards the solvent). The concentration of surfactantabove which micelle formation becomes appreciable is defined as thecritical micelle concentration, and is marked by a plateau of nearlyconcentration independent osmotic pressure and surface tension of thesolution.

As used herein, the term “partitioning species” refers to those species,other than the analyte of interest, which can migrate from the aqueousphase to the hydrophobic phase (or vice versa) in response to a changein pH in the aqueous phase and which substantially affect theconcentration dependent signal (i.e., the signal provided by theindicator component which is proportional to the concentration of theanalyte of interest in the medium being measured). A material is“substantially free” of partitioning species when the species are nolonger capable of substantially affecting the concentration dependentsignal.

As used herein, the term “response time” refers to the time necessaryfor the concentration dependent signal of a given sensor to reflect theconcentration of the analyte of interest when the sensor is exposed tothe medium containing the analyte. The response time includes any timenecessary for the sensor to stabilize to the medium, but does notinclude the time over which migration of partitioning species occurs tointroduce drift.

DETAILED DESCRIPTION

The sensors of the present invention comprise an emulsoid of a dispersedaqueous first phase in a hydrophobic second phase.

The sensors are formed from a stable emulsion of the dispersed aqueousfirst phase and a hydrophobic second phase precursor. Preferably, theaqueous first phase includes an indicator component in a buffersolution, and a water soluble emulsification enhancement agentpreferably comprising a nonionic amphipathic surfactant. Preferably, thehydrophobic second phase contains a polymeric precursor having a waterinsoluble emulsification enhancement agent comprising a plurality ofdispersed hydrophobic particles. The hydrophobic second phase ispermeable to the analyte of interest and impermeable to ionizedhydrogen.

The emulsion may contain humectants that may enhance the sensorproperties, e.g., imparting dry web stability, rapid rehydration, andresponse to atmospheric CO₂. The emulsion can be readily formed into avariety of shapes, as described herein, and is polymerized or hardenedto form an emulsoid.

In a presently preferred embodiment, a solution or dispersion of asuitable indicator dye is formed in an aqueous buffer. The aqueousmixture preferably further comprises a water soluble emulsificationenhancement agent comprising a nonionic amphipathic surfactant. Theaqueous mixture is then emulsified with (or uniformly dispersed orsuspended with) a liquid precursor of a polymeric material and a waterinsoluble emulsification enhancement agent preferably comprising aplurality of dispersed hydrophobic particles. During the emulsificationor suspension step, the aqueous phase is broken up into very smalldroplet sizes. The polymeric material is chosen such that the aqueousphase is not readily soluble in either the precursor materials for thepolymeric material or the polymerized polymeric material. Thus theaqueous phase always retains its integrity. By emulsifying or suspendingthe aqueous phase into the polymeric precursor materials, very smalldiscrete “micro-compartments” (alternatively referred to as “droplets”or “cells”) of the aqueous first phase can be formed in the polymericsecond phase. Upon curing or crosslinking of the polymeric phase, thesemicro-compartments are fixed in dispersed positions which areessentially uniformly scattered throughout the polymeric material. An“emulsoid” of the aqueous first phase is thus formed in the polymericsecond phase.

Preferably, the aqueous phase in this preferred embodiment is veryevenly distributed within the polymeric phase, when it is fixed inposition in the emulsoid, and its concentration is very evenlydistributed throughout the emulsoid. As a result, the concentration ofthe aqueous phase is uniform through the emulsoid, and the sensingcharacteristics of the gas sensor of the invention are also veryuniform.

Contrary to other gas sensors, by using very small emulsion sizedparticles, the surface area of the individual micro-compartments andthus the totality of the micro-compartments of the aqueous phase is verylarge. Because the surface area of the aqueous phase which is in contactwith the surface area of the polymeric phase is very large for the gassensors of this invention, gas exchange to the sensing aqueous phase isfast and is uniformly sensitive to the gas concentration within thepolymeric phase.

A suitable homogenizer such as a Vertishear homogenizer may be used toemulsify the mixture of the aqueous phase and the polymeric precursor.The emulsification enhancement agents contribute to the stability of theemulsion or suspension such that it has an increased shelf life. When itis desired to form the gas sensor of the invention, the cross-linkerand/or catalyst may be added (if they are not already present in thepolymeric precursor) or the sensor may be exposed to visible light or UVlight if a photosensitive initiator is present. The resulting mixturemay then be shaped and cured.

A very simple gas sensor can be formed by simply depositing a drop ofthe mixture of the emulsion and a cross-linking agent onto the end of afiber optic fiber and allowing it to cure into an emulsoid directly onthe end of the fiber. Alternatively, the emulsion mixture or a sheet ofsensor material formed from the emulsion as described in copending U.S.patent application “Sensing Elements and Methods for Making Same,” Ser.No. 08/159,799, can be placed in a sensor holder or “cassette” to form asensor.

Following emulsification, the aqueous phase is present in the polymericprecursor in micro-compartments which are of a size less than 125microns. Preferably the micro-compartments are nearly monodisperse andsmaller than 5 microns. More preferably, gas sensors of the presentinvention will have micro-compartments of the aqueous phase in thepolymeric phase wherein the majority of the population of thecompartments will be on the order of 2 microns. It is, of course,realized that the particles will actually be in a statistical range ofparticle sizes, some slightly larger than the above noted sizes, someslightly smaller, depending on the emulsification procedure, andapparatus.

The volume of the aqueous phase generally occupies between about 1 and80% of the sensing composition. More preferably, the aqueous phasegenerally occupies between about 10 and 60% of the sensing composition,and most preferably between about 15 and 40% of the sensing composition.

The stability of the emulsion may be assessed by measuring therheological properties of the emulsion as a function of time. Theinitial elastic or “storage” modulus of the uncured emulsion isgenerally greater than about 100 Pa and the equilibrium elastic modulusat 48 hours is also generally greater than 100 Pa when tested asdescribed herein. More preferred emulsions have an initial elasticmodulus greater than 200 Pa and an equilibrium elastic modulus at 48hours greater than 200 Pa. Most preferred emulsions have an initialelastic modulus greater than 300 Pa and an equilibrium elastic modulusat 48 hours greater than 300 Pa.

At a minimum, the aqueous phase must contain an indicator of the gas ofinterest for which the sensor is being used. In general, the aqueousfirst phase includes a suitable indicator component or “dye” in a buffersolution. The first phase preferably also includes a water solubleemulsification enhancement agent such as a nonionic amphipathicsurfactant.

Other materials can be incorporated into the aqueous phasemicro-compartments. Depending on the gas of interest, these othermaterials would be chosen to contribute to the operating characteristicsof the gas sensor. For example, additional materials (e.g., humectants)can be added to lower the vapor pressure of the aqueous phase in thepolymeric phase so as to retard the evaporation of the aqueous phaseduring formation of the gas sensor of interest. Aside from materialswhich contribute to the physical formation of the emulsoid of theaqueous phase in the polymeric phase, further additives can be added tothe aqueous phase for enhancement of the storage and/or operatingcharacteristics of the gas sensor as for instance osmoregulatory agents(e.g., NaCl) and/or bacteriostatic agents.

According to the present invention an indicator component or “dye” isutilized for sensing a gas of interest. Preferably, the indicatorcomponent is a pH sensitive optical indicator component. The dye can beone which acts with the gas of interest either by directly interactingwith the gas or by indirectly acting with the gas, as for example, bysensing a pH change in a medium wherein the pH change is caused byinteraction of the gas of interest with that medium. Interaction of thegas of interest with the dye, either directly or indirectly, can bemonitored by any suitable optical technique as for instance by eitherfluorescence or by absorption.

A particular gas of interest for the gas sensor of this invention iscarbon dioxide. Preferably for sensing carbon dioxide a pH sensitive dyewould be solubilized in the aqueous phase. Gas exchange through thepolymeric phase and into the aqueous phase solubilizes the carbondioxide gas in the aqueous phase as carbonic acid which interacts withthe buffer ions. The dye chosen is one which is responsive to theconcentrations or the ionic species of the carbonic acid in the aqueousphase, i.e., an acid-base responsive dye.

In choosing a dye for measuring carbon dioxide in blood, considerationis given to matching the pKa of the dye to the pH range of the aqueousphase induced by physical CO₂ levels. In constructing a gas sensor ofthis invention for use in sensing carbon dioxide gas in blood, we havefound that hydroxypyrene trisulfonate (“HPTS”) has characteristics whichare particularly desirable. HPTS, which is a known fluorescence dye forcarbon dioxide, has a relatively large “Stokes shift.” For use influorescence spectroscopy, this separates the excitation light from theemission light which improves the measurement of the emission light forimproved gas sensor performance. The hydroxypyrene trisulfonic acid canbe used as a free acid or as one of its salts as for instance an alkalior alkali earth salt.

Suitable indicator components for use in the present invention include:9-amino-6-chloro-2-methoxyacridine;2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein;2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethylester, 2′,7′-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein,acetoxymethyl ester; 5-(and -6)-carboxy-2′,7′-dichlorofluorescein;5-(and -6)-carboxy-2′,7′-dichlorofluorescein diacetate; 5-(and-6)-carboxy-4′,5′-dimethylfluorescein; 5-(and-6)-carboxy-4′,5′-dimethylfluorescein diacetate; 5-carboxyfluorescein;6-carboxyfluorescein; 5-(and -6)-carboxyfluorescein;5-carboxyfluorescein diacetate; 6-carboxyfluorescein diacetate;5-carboxyfluorescein diacetate, acetoxymethyl ester; 5-(and-6)-carboxyfluorescein diacetate; 5-(and -6)-carboxynaphthofluorescein;5-(and -6)-carboxynaphthofluorescein diacetate; 5-(and-6)-carboxySNAFL®-1, succinimidyl ester {5′(and6′)-succinimidylester-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};5-(and -6)-carboxySNAFL®-2, succinimidyl ester {5′(and6′)-succinimidylester-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNAFL®-1{5′(and6′)-carboxy-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNAFL®-1 diacetate {5′(and6′)-carboxy-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNAFL®-2 {5′(and6′)-carboxy-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNAFL®-2 diacetate {5′(and6′)-carboxy-9-chloro-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-1 {5′(and6′)-carboxy-10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-1, AM acetate{3-acetoxy-5′-acetoxymethoxycarbonyl-10-dimethylamino-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-2 {5′(and6′)-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-2, AM acetate{3-acetoxy-5′-acetoxymethoxycarbonyl-10-diethylamine-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-6 {5′(and6′)-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};carboxySNARF®-X {5′(and6′)-carboxy-3-hydroxy-tetrahydroquinolizino[1,9-hi]spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};5-chloromethylfluorescein diacetate; 4-chloromethyl-7-hydroxycoumarin;Cl-NERF{4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylicacid}; dextran, BCECF, 10,000 MW, anionic {dextran,2′,7′-bis(2-carboxyethyl)-5(and 6)-carboxy-fluorescein, anionic};dextran, BCECF, 40,000 MW, anionic; dextran, BCECF, 70,000 MW, anionic;dextran, Cl-NERF, 10,000 MW, anionic; dextran, Cl-NERF, 70,000 MW,anionic; dextran, Cl-NERF, 10,000 MW, anionic, lysine fixable; dextran,DM-NERF, 10,000 MW, anionic {dextran,4-[2,7-dimethyl-6-(ethylamino)-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid, anionic}; dextran, DM-NERF, 70,000 MW, anionic;dextran, DM-NERF, 10,000 MW, anionic, lysine fixable; dextran,7-hydroxycoumarin, 10,000 MW, neutral; dextran, 7-hydroxycoumarin,70,000 MW, neutral; dextran, β-methylumbelliferone, 10,000 MW, neutral;dextran, β-methylumbelliferone, 70,000 MW, neutral; dextran, SNAFL®-2,10,000 MW, anionic {dextran,9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]3′-one,anionic}; dextran, SNAFL®-2, 70,000 MW, anionic {dextran,10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one,anionic}; dextran, SNARF®-1, 10,000 MW, anionic; dextran, SNARF®-1,70,000 MW, anionic; 1,4-dihydroxyphthalonitrile; DM-NERF{4-[2,7-dimethyl-6-ethylamino)-3-oxo-3H-xanthen-9-yl]1,3-benzenedicarboxylic acid}; fluorescein diacetate;8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt;naphthofluorescein; naphthofluorescein diacetate; SNAFL®-1{3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one};and SNAFL®-1, diacetate{3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1′(3′H)-isobenzofuran]-3′-one}.Many of the above indicator compounds are commercially available fromMolecular Probes, Inc. “SNARF” and “SNAFL” are registered trademarks ofMolecular Probes, Inc. The structures of many of the aforementionedindicator compounds are listed in “Handbook of Fluorescent Probes andResearch Chemicals”, 5th Edition, pages 129 to 141 (1992) by Richard P.Haugland, which is herein incorporated by reference. Also absorptiondyes such as chlorophenol red, bromo cresol purple, nitrophenol, bromothymol blue, pinachorome and phenol red could be used.

Preferably, the concentration of the dye in the aqueous phase would befrom about 1 millimolar to about 20 millimolar with about a 2 to 8millimolar solution being typically used.

Certain properties of the emulsion or suspension between the aqueousphase and the polymeric precursor can be enhanced by adding additionalagents herein identified by the terminology “emulsification enhancementagents”. These emulsification enhancement agents enhance certainmanufacturing properties such as shelf life of the gas sensorintermediates by stabilizing the emulsion with respect to aggregationand coalescence. By stabilizing the emulsion or suspension of theaqueous phase and the polymeric precursor with respect to aggregation,it is not mandatory to immediately polymerize the aqueousphase-polymeric precursor emulsion or suspension into the emulsoid gassensor of the invention. With the addition of the emulsificationenhancement agents, the emulsion or suspension of the aqueous phase andpolymeric precursor is stable and can be set aside for formation intothe emulsoid gas sensor of the invention at a later time. This reducesthe need to adhere to a tight manufacturing schedule and reduces orprevents the generation of manufacturing “scrap materials” which areeconomically wasteful.

We have discovered that a particularly preferred emulsificationenhancement agent system comprises the combination of a water solubleemulsification enhancement agent and a water insoluble emulsificationenhancement agent. The water soluble emulsification enhancement agent ispreferably provided in the aqueous first phase and the water insolubleemulsification enhancement agent is preferably provided in thehydrophobic second phase.

Preferably, the water soluble emulsification enhancement agents areamphipathic copolymers or surfactants. More preferably, the watersoluble emulsification enhancement agents are nonionic, amphipathiccopolymers or surfactants.

In general, an amphipathic compound is a molecule capable of formingmicelles and generally consists of a chemical structural group (thelyophobic group) having very little affinity for the solvent (hence lowsolubility in that solvent), combined with a chemical structural group(the lyophilic group) having very high affinity for the solvent (hencehigh solubility in that solvent). When the solvent is water orpredominantly aqueous, the lyophobic group is described as hydrophobicand the lyophilic group is described as hydrophilic.

Preferred water soluble emulsification enhancement agents comprises anonionic, amphipathic copolymer containing both hydrophilic andhydrophobic moieties. More preferably, the hydrophilic moiety ispolyethylene oxide and the hydrophobic moiety is polypropylene oxide.

In one preferred embodiment, the water soluble emulsificationenhancement agent is an ABA block copolymer, wherein the A block is apolyethylene oxide moiety and the B block is a polypropylene oxidemoiety. Alternatively, the water soluble emulsification enhancementagent may be an AB, BA, or BAB block copolymer, wherein the A block is apolyethylene oxide moiety and the B block is a polypropylene oxidemoiety.

Suitable water soluble emulsification enhancement agents have a“hydrophile-lipophile balance” (HLB) of at least 5, when determined asdescribed herein. Preferred water soluble emulsification enhancementagents have a HLB of at least 7, more preferred water solubleemulsification enhancement agents have a HLB of at least 8, and mostpreferred water soluble emulsification enhancement agents have a HLB ofat least 10.

Preferred sensors are formed from emulsions in which the water solubleemulsification enhancement agent has a “cloud point” greater than 20° C.More preferred sensors are formed from emulsions in which the watersoluble emulsification enhancement agent has a “cloud point” greaterthan 60° C., and most preferably greater than 100° C.

Preferably, the water soluble emulsification enhancement agent ispresent in a concentration up to about 10 weight % in the sensingcomposition. More preferably, the water soluble emulsificationenhancement agent is present in a concentration of between 0.01 and 5weight % in the sensing composition, and most preferably, the watersoluble emulsification enhancement agent is present in a concentrationof between 0.1 and 3 weight % in the sensing composition.

Preferred water soluble emulsification enhancement agents include blockcopolymer surfactants such as “PLURONIC” and “PLURONIC R” sufactantsfrom BASF (Wyandotte, Mich.). Suitable block copolymer surfactants areprepared by the controlled addition of propylene oxide to the twohydroxyl groups of propylene glycol. Ethylene oxide is then added tosandwich this hydrophobe between hydrophilic groups. The generalstructure of commercially available block copolymers of this type(PLURONIC) is represented below:

HO—(CH₂CH₂O)_(x)—(CH₂C(CH₃)HO)_(y)—(CH₂CH₂O)_(x′)—H

Alternatively, copolymers may be prepared by reacting ethylene oxidewith ethylene glycol. Propylene oxide is then added to obtainhydrophobic blocks on the outside of the molecule. The general structureof commercially available block copolymers of this type (PLURONIC R) isrepresented below:

HO—(C(CH₃)HCH₂O)_(x)—(CH₂CH₂O)_(y)—(CH₂C(CH₃)HO)_(x′)—H

Suitable commercially available surfactants include ABA block copolymerssuch as PLURONIC surfactants, and BAB block copolymers such as PluronicR surfactants. Suitable PLURONIC surfactants include: L10, L35, L42,L44, L62, L62D, L62LF, L63, L64, L72, L77, L92, F38, F68, F68LF, F77,F87, F88, F98, F108, F127, P65, P75, P84, P85, P103, P104, P105, andP123. PLURONIC F108 is presently most preferred. Suitable PLURONIC Rsurfactants include: 10R5, 10R8, 17R4, 17R8, 25R4, 25R5, 25R8, and 31R4.

Also suitable are similar ABA and BAB surfactants sold under thetradenames Poloxamers (BASF); Hodags (Calgene Chemical); and Synperonics(ICI Chemicals).

Other suitable water soluble emulsification enhancement agents includepolyalkylene oxide-modified polydimethylsiloxane surfactants such as“SILWET” surfactants from OSi Specialties. In general, these blockcopolymer surfactants are composed of a siloxane molecular backbone withorganic polyalkylene oxide pendant groups. A major class of thesesurfactants is a linear polydimethylsiloxane to which polyethers havebeen grafted through a hydrosilation reaction. The general structure ofthese surfactants is represented below:

Me₃SiO(Me₂SiO)_(x)(Me(PE)SiO)_(y)SiMe₃

where:

PE is —CH₂CH₂CH₂O(EO)_(m)(PO)_(n)Z;

Me is methyl;

EO represents ethyleneoxy;

PO represents 1,2-propyleneoxy; and

Z can be either hydrogen or a lower alkyl radical.

Another class is a branched polydimethylsiloxane to which polyethershave been attached through condensation chemistry. These surfactantshave the general structure:

(MeSi)_(y-2)[(OSiMe₂)_(x/y)O—PE]_(y)

where:

PE is —(EO)_(m)(PO)_(n)R, and

R is a lower alkyl group.

By varying the coefficients x, y, m, and n, a broad range of surfactantsare obtained.

Suitable SILWET surfactants include: L77, L720, L7001, L7002, L7087,L7200, L7230, L7600, L7604, L7605, L7607, and L7657.

Other suitable water soluble emulsification enhancement agents includeblock copolymer surfactants comprising propylene oxide and ethyleneoxide blocks. The general structure of commercially available blockcopolymers of this type is represented below:

RO—(CH₂CH₂O)_(x)—(CH₂C(CH₃)HO)_(y)—H

where:

R is H or alkyl.

Representative examples of surfactants of this class include: HypermerB261 (PEO-PPO block copolymer), Dow P15-200 (random copolymer of PEO andPPO), and UCON 75H-90000 (random blocks of PEO and PPO).

Other suitable water soluble emulsification agents include nonionicsurfactants based on ethylene oxide. The general structure ofcommercially available surfactants of this type is represented below:

R—(CH₂CH₂O)_(x)—H

where:

R is H, OH, or alkyl, provided that x is at least 40, and preferably atleast 70, when R is H or OH.

Representative examples of water soluble EEAs of this class where R isOH, include polyethylene oxide. Suitable polyoxyethylene alcohols alsoinclude BRIJ surfactants available from ZENECA, formerly available fromICI Chemicals, such as BRIJ 35, BRIJ 68, BRIJ 97, BRIJ 99, BRIJ 700 andBRIJ 700s. Representative examples of water soluble EEAs of this classwhere R is alkyl, include polyoxyethylene sorbitan fatty esters (e.g.,polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitanmonopalmitate, and polyoxyethylene sorbitan monostearate) such as TWEENsurfactants available from Zeneca. Suitable TWEEN surfactants includeTWEEN 20 (polyoxyethylene (20) sorbitan monolaurate), TWEEN 40, TWEEN60, and TWEEN 80.

Suitable water soluble emulsification enhancement agents have a weightaverage molecular weight between 100 and 50,000. Preferred water solubleemulsification enhancement agents have a weight average molecular weightbetween 500 and 20,000, and more preferred water soluble emulsificationenhancement agents have a weight average molecular weight between 5,000and 20,000.

The aqueous phase preferably includes a suitable buffer. For example,aqueous compositions can be prepared by addition of various amounts ofindicator, sodium bicarbonate, and a 50:50 mix of monobasic sodiumphosphate and dibasic sodium phosphate. Those skilled in the art willrecognize that these same compositions can be prepared in alternate wayswithout affecting the resultant buffer composition or the resultantbuffer performance as a function of CO₂ partial pressure. Furthermore,it is recognized that the sodium ion or chloride ion as counterions forthe buffering species can be replaced by other salt forming ions withoutchanging the scope or intent of this invention.

Preferred for use in sensing carbon dioxide is a bicarbonate ion basedbuffer in the aqueous phase. Such a buffer can be chosen so as to have abuffer range compatible with the response range of the dye. Such a rangemight, for instance, mimic the physiological pH range of blood. Suitablefor the preparation of such a bicarbonate ion buffer would be sodiumbicarbonate, sodium carbonate and sodium hydroxide or other suitablebuffer agents. Suitable dispersed aqueous first phases are buffered to apH range between about 5 and 14. For measuring blood carbon dioxide withhydroxypyrene trisulfonic acid, a pH range between about 7 and 8 is themost desirable.

Preferably, the concentration of the sodium bicarbonate and HPTSindicator should be chosen to optimize the sensitivity of the sensorover the range of CO₂ partial pressures commonly encountered duringblood gas monitoring. In addition, this optimized sensitivity can beobtained at a higher sodium bicarbonate concentration by increasing thepKa of the indicator.

Generally the concentration of the phosphate buffer in the aqueous phasewould be from about 1 millimolar to about 50 millimolar with about a 10millimolar solution typically being used. Generally the concentration ofthe bicarbonate buffer in the aqueous phase would be from about 5millimolar to about 200 millimolar, with about a 20 millimolar formalconcentration being used.

The aqueous phase also preferably includes a suitable humectant.Although not contributing essentially to the stability of the emulsion,the addition of a humectant to the dispersed aqueous phase may addadditional improvements to the cured emulsoid which include improvedintensity stability in the dry coated web, improved rehydration rate,improved autoclave stability, and improved manufacturing robustness. Inone embodiment, the humectant constitutes the major component of thedispersed phase.

Preferred humectants may provide sensors having a shorter equilibrationtime when the sensor is moved from a dry environment to a humid oraqueous environment compared to a sensor that does not contain ahumectant. In addition, more preferred humectants are thermally stableat temperatures up to 121° C. for 1 hour.

In general, a humectant is a substance that has an affinity for waterand provides a stabilizing action on the water content of a material orcomposition. Suitable humectants may vary in molecular weight, chemicalcomposition, heat stability, and purity. Examples of preferredhumectants include hydroxypropyl starch, hydroxyethyl starch, dextran,polyvinylalcohol, glycerol, polyvinylpyrrolidone, xanthan gum, gumarabic, methyl cellulose, tragacanth, acacia, agar, pectin, sodiumalginate, alginate derivatives, proteins, gelatins, guar gum,polyethylene glycol, polyethylene oxide, and hydrogels.

Suitable commercially available humectants include: Starpol 530 (ahydroxypropyl-substituted polysaccaride with M_(w) ^(˜)500,000 to600,000), Starpol 560 (a hydroxypropyl-substituted polysaccaride withM_(w) ^(˜)900,000 and comprising a mixture of 27 parts amylose and 73parts amylopectin), glycerol, trehalose, xanthan gum, etc.

Suitable humectants include water soluble molecules having a weightaverage molecular weight below about 4 million. Preferred humectantsinclude water soluble molecules with a weight average molecular weightbelow 2 million, and more preferably those molecules having a weightaverage molecular weight below 1,000,000.

The amount of humectant in the sensor's dispersed phase is preferablychosen to provide the desired additional improvements to the emulsion.In one embodiment, the humectant is present in an amount between about0.5 and 99% by weight of the dispersed phase. More preferably, forsensors used in a wet medium, such as blood, the humectant is present inan amount between 1 and 70% by weight of the dispersed phase, and mostpreferably, the humectant is present in an amount between 5 and 30% byweight of the dispersed phase. Preferably, for sensors used in a drymedium, such as air, the humectant is present in an amount at least 1%by weight of the dispersed phase, more preferably, the humectant ispresent in an amount at least 30% by weight of the dispersed phase, mostpreferably, the humectant is present in an amount at least 50% by weightof the dispersed phase, and optimally, the humectant is present in anamount at least 70% by weight of the dispersed phase.

A hydrophobic second phase (e.g., a polymeric phase) is chosen as acarrier for the dispersed aqueous phase and to maintain the individualmicro-compartments of the first phase in their dispersed form. Thesecond phase should be permeable to the gas of interest. It should alsobe translucent or transparent to the wavelength or wavelengths of lightutilized in the measurement of the gas of interest. Further, since it isnecessary to maintain the aqueous phase isolated from the carrier fluidof the gas of interest, the second phase should be substantiallyimpermeable to liquid water. In order to isolate the indicator and/orany other ingredients in the aqueous phase, the hydrophobic phase shouldalso be impermeable to ionic species.

Because of their high gas permeability and ionized hydrogenimpermeability, silicone polymers are preferred for use as thehydrophobic secondary phase which separates the aqueous phase and themedium being analyzed. Care should be taken to select appropriatematerials for use as the hydrophobic phase or to treat the materials toremove or immobilize any partitioning species that might contribute toundesirable drift.

In general, the polymeric phase can be prepared via severalpolymerization reactions. In addition to traditional “addition type”polymerization, the polymeric phase can be prepared using free radicalpolymerization reactions (e.g., using silicones having ethylenicallyunsaturated groups); condensation polymerization reactions (e.g., usingsilanol terminated silicones cross-linked with alkoxyl silanes usingcatalysts such as tin derivatives); or photoinitiated polymerizationreactions (e.g., using ultraviolet or visible light catalysts).

In one presently preferred embodiment the polymeric phase is preparedvia a photoinitiated polymerization reaction and optionally followedwith a thermal polymerization reaction. This may be done using eitherUV, near IR, or visible light. In one embodiment a free-radicalinitiator is utilized to crosslink an acrylate or methacrylatefunctional silicone polymer. Alternatively, a radiation activatedhydrosilation reaction may be employed (as described in U.S. Pat. Nos.4,530,879, 4,510,094, and 4,916,169, which are herein incorporated byreference) with traditional siloxane polymers and crosslinkers.

For use in forming a carbon dioxide gas sensor, polydimethysiloxane,used in conjunction with a cross-linking agent and a platinum catalystsuch as a Karstedt catalyst, is particularly preferred. Alternatively, aphoto-activated catalyst, such as are described in U.S. Pat. Nos.4,916,169; and 5,145,886, may be used and may result in a decrease inscrap and waste compared to typical silicone catalyst formulations.Photo-activated systems are also preferred for their greater flexibilityin manufacturing. Traditional silicone systems require careful attentionto working time and setting time constraints. Great care must be takento fully form the sensor within the working time of the siliconematerial. Failure to finish forming the sensor within the allotted timeresults in scrap product. Photo-activated materials are more convenient,since the activation step can be delayed until the sensor is completelyand fully formed. This virtually eliminates waste due to prematuresetting.

Suitable photoinitiators for use in the present invention should notundesirably contribute to CO₂ conditioning drift or undesirablyinterfere with the transmission of either the excitation light signal orthe emission light signal through the sensor.

Preferably, from about 1 gram to about 4 grams of the aqueous solutionwould be added to about 10 grams of the polymeric precursor. Morepreferably, about 2 to 3 grams of the aqueous phase per 7 to 10 grams ofthe polymeric precursor is used. Preferably, the cross-linking agentwould be added from about 2% to about 20% by weight of the polymericprecursor with approximately 5% to 10% by weight with respect to theweight of the polymeric precursor typically being used.

As previously mentioned, the hydrophobic phase preferably comprises awater insoluble emulsification enhancement agent. The water insolubleemulsification enhancement agent preferably comprises a plurality ofdispersed hydrophobic particles. Such agents serve to stabilize theinitially formed emulsion or dispersion prior to final crosslinking orcure. These agents, when added to the hydrophobic phase, may also serveto enhance the structural characteristics of the hydrophobic phase aftercrosslinking. That is to say, the filler may serve to improve themechanical strength or integrity of the cured matrix.

While not wishing to be bound by any particular theory as to themechanism, we believe that the amphipathic nature of the water solubleemulsifier allows the emulsifier to orient at the water/oil interfaceand interact with the water insoluble emulsifier which is also orientedat that interface, thus leading to enhanced aggregation stability.

Suitable hydrophobic particles include organic or inorganic particles.Suitable particles include hydrophilic particles that have been treatedwith an agent or agents to render the surface hydrophobic. Suitableinorganic particles include aluminum oxide, zirconium oxide, titaniumoxide, and silica. Preferred inorganic particles are treated and includefumed, precipitated, or finely divided silicas.

More preferred inorganic particles include the colloidal silicas knownunder the tradenames of CAB-O-SIL (available from Cabot) and AEROSIL(available from Degussa).

Suitable inorganic fillers include surface treated colloidal silicafillers such as CAB-O-SIL TS-530, and TS-720; and Degussa R812; R812S,and R202. “CAB-O-SIL TS-530” is a high-purity treated fumed silica whichhas been treated with hexamethyldisilazane (HMDZ). The treatmentreplaces many of the hydroxyl groups on the fumed silica withtrimethylsilyl groups. As a result the silica is made hydrophobic. Thesurface area of the TS-530 material is approximately 200 m²/g±40 m²/g(using BET method).

Preferably the hydrophobic filler particle has a relativehydrophobicity, as described and measured herein, of at least 2, morepreferably at least 4, and most preferably at least 5.

Preferably the hydrophobic filler particle is a finely-divided,substantially water insoluble solid, with a mean volume average diameterless than 5 microns, the surface of which may be treated with an organiccompound. Suitable filler particles exhibit a low affinity for water, asreflected by a solid/liquid contact angle (measured through the aqueousphase) of greater than 90 degrees. More preferred hydrophobic particleshave a solid/liquid contact angle (measured through the aqueous phase)greater than 110 degrees, and most preferred hydrophobic particles havea solid/liquid contact angle greater than 130 degrees.

Preferably, the radius of curvature of the hydrophobic particle is lessthan the radius of curvature of the emulsion droplet. More preferredhydrophobic particles have a mean volume diameter less than 5 microns,and most preferred hydrophobic particles have mean volume diameter lessthan 1 micron.

In one embodiment, the hydrophobic particles are chemically bonded tothe hydrophobic second phase. This may be accomplished, for example, bybonding the silicone chains to the silica in the presence of an ammoniacatalyst under heat and vacuum. Alternatively, hexamethyldisilazane canbe used.

Preferably, the filler is selected such that undesirable partitioningspecies are not inadvertently brought into the sensor composition. Forexample, Tullanox 500 fumed silica has been observed to contain asignificant level of a basic species (introduced with a hydrophobicsurface treatment). The residual base (believed to be ammonia) likelyleaves the sensor during storage or during processing of the sensor.Nevertheless, as an extra precaution, one may preferably use fillerwhich has been heated under vacuum (e.g., for 12 hours at 150° C. and at2 mm Hg) or a deammoniated filler such as CAB-O-SIL TS-530. Alsopreferred are silica fillers which have been hydrophobically treated byprocesses which do not result in the presence of basic impurities (suchas CAB-O-SIL TS 610 or CAB-O-SIL TS 720).

Preferably, the hydrophobic particles are present in a concentration ofbetween 0.1 and 20 weight % in the sensing composition. More preferably,the hydrophobic particles are present in a concentration of between 1and 10 weight % in the sensing composition, and most preferably between3 and 6 weight % in the sensing compound.

Certain materials used in traditional CO₂ sensors, for example, in theaqueous first phase, in the hydrophobic second phase, or in other partsof the sensor, are believed to undesirably contribute to CO₂conditioning drift and/or saline drift. These materials may themselvescontribute to drift or contain “impurities” or residual species(hereinafter collectively referred to as “partitioning species”) thatcontribute to the drift problem. The amount of such materials orimpurities needed for drift to occur is extremely small. Because of thelarge number of ingredients and materials that go into a typical CO₂sensor, titratable partitioning species are ubiquitous unlessextraordinary precautions are taken to eliminate them or control them.Each and every part of the sensor must be considered for its potentialcontribution of titratable partitioning species. This includes theaqueous phase (including, for example, surfactants used therein), thehydrophobic phase (including, for example, the silicone polymer and/orcross-linker, and fillers used therein), any optional films or overcoats(for example, substrate films or webs used when coating sensingelements, optical barrier films, etc.), and any optional adhesivesand/or adhesion promoters used to secure the sensor to an optical fiber,cassette, or a substrate film. The partitioning species may also beliberated or released from one or more of the sensor components as aresult of a subsequent process (e.g., a heating process or autoclaving)or exposure to an environment. For example, some materials containedwithin traditional sensors contain species which are believed to becomepartitioning species only when the sensor is steam sterilized or heated.With this understanding of the causes of CO₂ conditioning drift andsaline drift several methods are proposed to provide sensors which areessentially drift free. By careful selection and/or purification ofcomponents essentially drift-free sensing elements are provided whichare not adversely affected by steam sterilization. A more completediscussion of the causes of CO₂ conditioning drift and saline drift isfound in copending U.S. patent application Ser. No. 08/375,304.

Other particular gasses of interest for the gas sensor of this inventioninclude ammonia, SO₂, and NO₂. For sensing ammonia, a pH sensitive dyewould be solubilized in the aqueous phase. Gas exchange through thepolymeric phase and into the aqueous phase solubilize the ammonia gas inthe aqueous phase which interacts with the buffer ions. The dye chosenis one which is responsive to the concentrations of the ionic species ofthe ammonia in the aqueous phase, i.e., an acid-base responsive dye.Preferred for use in sensing ammonia is an ammonium chloride ion basedbuffer in the aqueous phase. Such a buffer can be chosen so as to have abuffer range compatible with the response range of the dye. Suitableindicators for use in measuring ammonium concentration include acridineorange, 1-hydroxypyrene-3,6,8-trisulphonate, and1-naphthol-4-sulphonate.

In one embodiment, this invention is directed to a gas sensor which canbe utilized with a fiber optical cable, i.e., a single optical fiber ora bundle of the same. Preferred sensors have a response time less than 5minutes, more preferably less than 2 minutes, and most preferably lessthan 1 minute. The fiber optic cable is associated with appropriateoptical and electronic devices for imposing an optical signal from thegas sensor. A plurality of techniques for transmitting and readingappropriate optical signals can be utilized with the gas sensors of theinvention. The optics and electronics for gas sensing will not bereviewed in detail, reference being made to the disclosures of U.S. Pat.Nos. RE 31,879; 4,557,900; and 4,824,789, which are herein incorporatedby reference. Notably, other means of transmitting light to and from thesensor may be employed. For example, a light source such as a LED may beplaced next to or against the sensor.

In another embodiment, this invention is directed to a gas sensorcomprising a sensor “cassette” through which a medium such as blood canflow. The cassette can be utilized with a fiber optical cable associatedwith appropriate optical and electronic devices, as discussed above, forimposing an optical signal from the gas sensor. U.S. Pat. Nos. 4,640,820and 4,786,474 (Cooper) describe a suitable cassette, and are hereinincorporated by reference. Notably, other means of transmitting light toand from the cassette may be employed. For example, a light source suchas a LED may be placed next to or against the sensor or cassette.

Seen in FIG. 1 is a drop 10 of the emulsion or suspension of the aqueousphase in the polymeric precursor. As is evident, the micro-compartments12 are dispersed in a uniform manner through the drop 10 of theemulsion.

For formation of a very simple gas sensor 14 of this invention, in FIG.2a, a drop of the above mixture is placed on the distal end 15 of anoptical fiber 16. The mixture of the cross-linking agent and thepolymeric precursor having the aqueous phase as an emulsion thereincures into an emulsoid 18 of the micro-compartments 20 of the aqueousphase in the polymeric material or carrier body 22. If desired, theemulsoid 18 can be retained on the end of the fiber 16 using a suitableoptional sleeve 24. The sleeve 24 can be constructed from a suitablematerial such as Teflon or the like. Further, to avoid light intensitychanges caused by factors other than the changes in partial pressure ofthe gas sensed, an overcoat 26 can be added as a layer over the exposedpositions of the emulsoid 18. For use with a fluorescent dye, theovercoat 26 is chosen to be opaque to the excitation light wavelengthλ_(ex) and to the emission light wavelength λ_(em) both of which aretransmitted along the same single optical fiber 16. A suitable materialfor the overcoat 26 would be vinyl end-capped poly(dimethyl) siloxaneimpregnated with carbon black.

As is evident in FIG. 2a, the size of the gas sensor 14 is dictated onlyby the optical fiber size. The gas sensor 14 thus formed is of asufficiently small size so as to be introducible directly into thecardiovascular system of a patient for direct real time measurement ofthe partial pressure of a blood gas such as carbon dioxide. If the fiberoptic fiber 16 of FIG. 2a is typically about 125 micron in diameter, itis evident that the emulsoid 18 is approximately equal to or less thanthis size in each of its orthogonally oriented width, height and depthdimensions. Other constructions of gas sensors are also possibleutilizing the emulsoid of this invention. It, of course, being realizedthat smaller sensors could be constructed by utilizing a smallerdiameter fiber optic cable.

By using the above noted gas sensor construction in conjunction withHPTS as a pH sensitive dye, determination time of the carbon dioxidepartial pressure is made in a time period of approximately one minute.This gas sensor preferably can be autoclaved to sterilize it withoutdetracting from or degrading its performance and during its use it isessentially temperature stable.

FIG. 2b shows a sensor 110 according to the present invention. Sensor110 is adapted to determine the concentration or partial pressure ofcarbon dioxide in blood. An optical fiber 112 is connected to anappropriate light transmitting apparatus 114, which is capable oftransmitting light at 410 and 460 nanometers. The light transmittingapparatus 114 generates the excitation light at these wavelengths. Theoptical fiber 112 is also connected to a light receiving apparatus 116,which, in turn, is connected to a conventional electronic processor 117.Located on the optical surface 118 of the optical fiber 112 is a matrix120 which is a carbon dioxide permeable material, such as a cross-linkedaddition cured siloxane polymer. Within the matrix 120 is a plurality ofmicro-compartments 121 comprising an aqueous phase including HPTSindicator dye. The highly carbon dioxide permeable matrix 120 adheres tothe optical surface 118 and slightly down along the sides 122 of the endof fiber 112. An opaque overcoat 124, comprising iron oxide pigmentdispersed in an addition cured polysiloxane, can then be applied overthe totality of the matrix 120 and down further along the side 122 ofthe fiber 112.

In use, sensor 110 functions as follows. The tip of optical fiber 112including matrix 120 and overcoat 124 is exposed or immersed in blood,the carbon dioxide concentration of which is to be determined. Lighttransmitting apparatus 114 transmits light at 410 nanometers to theoptical fiber 112. The excitation light at 410 nanometers causes thematrix 120 to fluoresce at 510 nm. In this case, the 410 nm light isabsorbed primarily by the acidic form of HPTS. Excited statedeprotonation follows, giving rise to 510 nm emission from the basicform of the dye. This emission is proportional to the amount of HPTSinitially present in the acidic form. As the concentration of carbondioxide in the blood increases, the pH of the aqueous phase drops andthe intensity of 510 nm emission associated with 410 nm excitationincreases. Light transmitting apparatus 114 then transmits light at 460nm to the optical fiber. The excitation light at 460 nm also causes thematrix 120 to fluoresce at 510 nm. In this case, the 460 nm light isabsorbed by the basic form of HPTS, which emits directly at 510 nm. Thisemission is proportional to the amount of dye initially present in thebasic form. As the concentration of carbon dioxide in the bloodincreases, the intensity of 510 nm emission associated with the 460 nmexcitation decreases. The fluorescent emitted signals are transmittedfrom matrix 120 through optical fiber 112 to light receiving apparatus116. Processor 117 uses information received by light receivingapparatus 116 on the longer emitted signal to determine a value of thecarbon dioxide concentration in the blood. Receipt and analysis of thisfluorescent light by light receiving apparatus 116 and processor 117 maybe carried out in a manner similar to that described in U.S. Pat. Nos.RE 31,897 and 4,557,900. Processor 117 uses information received bylight receiving apparatus 116 of the fluorescent signals emitted at 510nanometers to develop a ratio of the emitted fluorescent signalassociated with 460 nm excitation to the fluorescent signal associatedwith 410 nm excitation. Using this ratio together with the above-notedcarbon dioxide concentration, processor 117 can determine a correctedconcentration of carbon dioxide in the blood to be analyzed. Thiscorrected carbon dioxide concentration is found to be accurate even ifthe optical fiber 112 is bent at one or more points along its lengthand/or if other light transmission difficulties are encountered.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous carbon dioxideconcentration results. Of course, the transmission of the excitation at460 nanometers can take place before transmission of the excitation at410 nanometers. Also, by proper selection of the optical indicators,e.g., fluorescent dyes, the concentration of other components ofinterest can be determined. In addition, media other than blood can beanalyzed.

The optical fiber 112 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring of oxygen concentration in the blood.Alternately, the present sensor can be embodied in a flow-throughhousing as shown, for example, in U.S. Pat. No. 4,557,900, to provideextra corporeal monitoring of carbon dioxide concentration in the blood.

FIG. 2c shows a sensor 210 according to the present invention. Sensor210 is adapted to determine the concentration or partial pressure ofcarbon dioxide in blood. An optical fiber 212 is connected to anappropriate light transmitting apparatus 214, which is capable oftransmitting light at 543 nanometers. The light transmitting apparatus214 generates the excitation light at this wavelength. The optical fiber212 is also connected to a light receiving apparatus 216, which, inturn, is connected to a conventional electronic processor 217. Locatedon the optical surface 218 of the optical fiber 212 is a matrix 220which is an carbon dioxide permeable material, such as a cross-linkedaddition cured siloxane polymer. Within the matrix 220 is a plurality ofmicro-compartments 221 comprising an aqueous phase including a dye(e.g., SNARF-6). The highly carbon dioxide permeable matrix 220 adheresto the optical surface 218 and slightly down along the sides 222 of theend of fiber 212. An opaque overcoating 224, comprising iron oxidepigment dispersed in an addition cured polysiloxane, can then be appliedover the totality of the matrix 220 and down further along the side 222of the fiber 212.

In use, sensor 210 functions as follows. The tip of optical fiber 212including matrix 220 and overcoating 224 is exposed or immersed inblood, the carbon dioxide concentration of which is to be determined.Light transmitting apparatus 214 transmits light at 543 nanometers tothe optical fiber 212. The excitation light at 543 nanometers causes thematrix 220 to fluoresce at two separate wavelengths. The emission at theshorter wavelength is associated with the acidic form of the indicator.The emission at the longer wavelength is associated with the basic formof the indicator. As the concentration of carbon dioxide in the bloodincreases, the pH of the aqueous compartment drops and the intensity ofthe short wavelength emission increases while the intensity of the longwavelength emission drops. Typically, the short wavelength emission ismeasured at 580 nm and the longer wavelength emission is measured at 630nm. Both the emissions at 580 nanometers and 630 nanometers aredependent on the concentration of carbon dioxide in the blood. Thefluorescent emitted signals are transmitted from matrix 220 throughoptical fiber 212 to light receiving apparatus 216. Processor 217 usesinformation received by light receiving apparatus 216 on the shorteremitted signal to determine a value of the carbon dioxide concentrationin the blood. Receipt and analysis of this fluorescent light by lightreceiving apparatus 216 and processor 217 may be carried out in a mannersimilar to that described in U.S. Pat. Nos. RE 31,897 and 4,557,900.Processor 217 uses information received by light receiving apparatus 216of the fluorescent signal emitted at 580 nanometers to develop a ratioof the emitted fluorescent signal at 580 nanometers to the fluorescentsignal at 630 nanometers. Using this ratio together with the above-notedcarbon dioxide concentration, processor 217 can determine a correctedconcentration of carbon dioxide in the blood to be analyzed. Thiscorrected carbon dioxide concentration is found to be accurate even ifthe optical fiber 212 is bent at one or more points along its lengthand/or if other light transmission difficulties are encountered.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous carbon dioxideconcentration results. Of course, the detection of the emission at 580nanometers can take place before detection of the emission at 630nanometers. Also, by proper selection of the optical indicators, e.g.,fluorescent dyes, the concentration of other components of interest canbe determined. In addition, media other than blood can be analyzed.

The optical fiber 212 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring of oxygen concentration in the blood.Alternately, the present sensor can be embodied in a flow-throughhousing as shown, for example, in U.S. Pat. Nos. 4,557,900, 4,640,820,and 4,786,474, to provide extra corporeal monitoring of carbon dioxideconcentration in the blood.

An alternate embodiment, which is described with reference to FIG. 2d,involves a sensor apparatus making use of intensity modulated (sinewave) signals in the MHz range. In this embodiment, sensor 310 isadapted to determine the concentration or partial pressure of carbondioxide in blood. An optical fiber 312 is connected to an appropriatelight transmitting apparatus 314, which is capable of transmittingintensity modulated (sine wave) light in the MHz range. The lighttransmitting apparatus 314 generates the modulated excitation light atthis frequency. The optical fiber 312 is also connected to a lightreceiving apparatus 316, which, in turn, is connected to a conventionalelectronic processor 317. The light transmitting apparatus 314 includesa frequency generator (one or more frequencies simultaneously) linked toan electrically controlled light emitting structure, such as a lightemitting diode, a frequency doubled light emitting diode, or acombination of elements such as a continuous wave laser or incandescentlight source coupled to an acoustooptic modulator or electroopticmodulator, and the like. The light receiving apparatus 316 includes ahighly sensitive light detector having a rapid response time. Suitabledetectors include photomultiplier tubes such as those sold under thetrademark R928 by Hamamatsu Photonics K.K., Hamamatsu, Japan, as well asavalanche photodiodes and microchannel plates, also available from thesame supplier. Using techniques well known in the art, heterodynedetection can be implemented by modulating the detector sensitivity at afrequency, equal to the fundamental modulation frequency, F_(f) in theMHz regime, plus or minus a heterodyne modulation frequency F_(h) in theHz or kHz region. The processor 317 may include, for example, an analogto digital converter coupled by a direct memory access device to acomputer, or an analog phase comparator circuit known to those skilledin the art, and the like. The SLM 48000MHF Fourier TransformSpectrofluorometer manufactured by SLM-Aminco in conjunction with a HeNelaser provides frequency modulated light generation, light receivingapparatus and processor capability to perform the methods outlinedherein; to measure phase shifts, demodulation factors, or both at eithera single modulation frequency or simultaneously at several modulationfrequencies. Commercial software is available to apply a well-knowndigital fast Fourier transform to the data and to interpret phase anddemodulation data at multiple modulation frequencies in terms of adistribution of emission lifetimes and relative contributions.

Located on the optical surface 318 of the optical fiber 312 is a matrix320 which is an carbon dioxide permeable material, such as across-linked addition cured siloxane polymer which is similar to thepolymer described previously and containing a plurality ofmicro-compartments 321 comprising an aqueous phase comprising, forexample, SNARF-6 dye (or any other suitable lifetime based pHindicator). The highly oxygen permeable matrix 320 adheres to theoptical surface 318 and slightly down along the sides 322 of the end offiber 312. An opaque overcoating 324, comprising iron oxide pigmentdispersed in an addition cured polysiloxane, can then be applied overthe totality of the matrix 320 and down further along the side 322 ofthe fiber 312.

In use, sensor 310 functions as follows. The tip of optical fiber 312including matrix 320 and overcoating 324 is exposed or immersed inblood, the carbon dioxide concentration of which is to be determined.Light transmitting apparatus 314 transmits light at 50 MHz and 543 nm tothe optical fiber 312. This excitation light causes the matrix 320 tofluoresce at 610 nm, an isobestic point for emission from the acid andbase forms of SNARF-6. The fluorescent emission is sine wave modulated.The emission lifetime for the acidic form of the dye is longer than theemission lifetime of the basic form of the dye. As the concentration ofcarbon dioxide in the blood increases, the pH of the aqueous compartmentdrops and the phase shift increases while the demodulation factordecreases.

The fluorescent emitted signal is transmitted from matrix 320 throughoptical fiber 312 to light receiving apparatus 316. Processor 317 usesinformation received by light receiving apparatus 316 on the emittedsignal to determine the extent of the phase shift and/or thedemodulation factor of this emitted signal. The extent of this phaseshift and/or this demodulation factor is dependent on the concentrationof carbon dioxide in the blood. Thus, by determining the extent of thisphase shift and/or this demodulation factor, values of the carbondioxide concentration in the blood can be obtained. Transmission,receipt and analysis of this modulated signal by light transmittingapparatus 314, light receiving apparatus 316 and processor 317 may becarried out using equipment and in a manner similar to that described inU.S. Pat. No. 4,840,485, which is incorporated herein by reference.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous carbon dioxideconcentration results. Of course, by proper selection of the opticalindicators, e.g., fluorescent dyes, the concentration of othercomponents of interest can be determined. In addition, media other thanblood can be analyzed.

The optical fiber 312 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring. Alternately, the present sensor can beembodied in a flow-through housing as shown, for example, in theabove-referenced Heitzmann patent, to provide extra corporealmonitoring. In addition, the light transmitting apparatus 314 and/orlight receiving apparatus 316 may be embodied in the flow-throughhousing without an intermediate optical fiber.

Seen in FIG. 3 is a schematic representation of an alternative CO₂sensor of the present invention. In this embodiment the gas sensor 30comprises a “cassette” or sensor holder 32 having a well 34. The well 34is open at one end, includes a bottom end wall 36 and a side wall 38. Adrop of emulsion 10 is placed in the well 34 and cured to form anemulsoid 40. An opaque layer 42 can be added as a layer over the exposedpositions of the emulsoid 40. In operation, a medium such as blood isbrought in contact with the exposed position of the emulsoid 40 (oralternatively in contact with the opaque layer 42). An excitation signalis transmitted through an optical fiber 44 which causes an emissionsignal from the indicator component. Alternatively, instead of using anoptical fiber 44 to transmit the excitation and emission signals, onemight either directly embed an LED and/or photodetector in the cassetteor place an LED and/or photodetector in contact with the cassette (notshown).

FIGS. 4 and 5 illustrate the use of a sensing element which may beproduced as described in U.S. patent application Ser. No. 08/159,799,which is herein incorporated by reference.

As shown in FIG. 4, this individual sensing element 50 is placed intowell 54 containing a transparent, silicone-based adhesive 57. Well 54 isopen at one end, includes a right circular cylindrical side wall 55 anda circular bottom end wall 58. The size of well 54 is such that theindividual sensing element 50 and silicone-based adhesive layer 57completely fill the well. Individual sensing element 50 is placed inwell 54 so that the transparent web layer 64 faces the bottom end wall58 of well 54. The opaque layer 62 includes an exposed surface 63 whichis raised relative to the inner surface 70 of sensor holder 56. Theopaque layer 62 substantially shields sensing composition layer 66 fromdirect contact with the medium, e.g., blood, to be monitored. Dependingon the specific sensing application involved, the exposed surface of theopaque layer can be recessed relative to, or flush with, the innersurface of the sensor holder.

Referring now to FIG. 5, in use sensor holder 56, made of a transparentpolycarbonate material, is placed in abutting relation to optical fiber72. Optical fiber 72 provides excitation light of appropriate wavelengthfrom light transmitting apparatus 74 to excite the sensing component inthe sensing composition layer 66 to fluoresce and provide a signalcharacteristic of the concentration of carbon dioxide located in themedium in contact with the opaque film 62. This optical fiber 72 alsotransmits the signal which is emitted from the sensing component andpasses such signal to a light receiving apparatus 76, which processes oranalyzes this emitted signal, e.g., as described in U.S. Pat. No.RE31,879, 4,557,900, and/or copending U.S. patent application Ser. Nos.08/136,967 and 08/137,289 to determine the concentration of carbondioxide in this medium.

Methods for Assessing Emulsion Stability

Various methods have been used for assessing the stability of emulsions,as reviewed by Tadros and Vincent (Encyclopedia of Emulsion Technology,Vol. 1, P. Becher, Ed., New York, 1983, pp. 129-285). Preferred methodsfor assessing the stability of concentrated emulsions include microscopeexamination and rheological characterization of the emulsion as afunction of time after homogenization.

The microscopic structure of emulsions was determined using a Zeissstandard 14 microscope with an Illuminator 100 halogenlamp/mercury/xenon and fluorescence source. The microscope was equippedwith a Zeiss MC 63 camera, a calibrated 10× eye piece, and 16× and 40×objectives. Thin smears of emulsion samples were made on glass slidesand mounted with a coverslip. The initial size distribution of emulsiondroplets was noted and optionally recorded photographically.

Emulsion stability over a 48 hour period was quantified by measuring thedynamic viscoelastic properties of the emulsion using a Bohlin VORcontrolled strain rheometer as described by Tadros (Colloids & Surfaces,Physiochemical and Engineering Aspects, 91 (1994) pp 50-55). Dynamicmeasurements as a function of time are often used to characterizeemulsion stability because this method uses low strains and is thereforenondestructive. In this case, initial, 24 hour and 48 hour values of theelastic or “storage” modulus (G′) and complex viscosity (η*) weremeasured. In general, stable emulsions are characterized by high elasticmodulus (greater than 100 Pa). Stability is further reflected by littleor no percentage change in elastic modulus with time.

Prepared samples (3 ml) were carefully loaded into a concentric cylindergeometry with a cup diameter of 16.5 mm and a bob diameter of 14 mmwhich results in a gap of 1.25 mm. To minimize wall slip effects aserrated bob was used in these measurements. An 11 g cm or 40 g cmtorque bar was used. All emulsions were subjected to an initial strainsweep at 0.001 to 0.20 radians (at 20 Hz) to help eliminate prior shearand loading effects on the emulsion microstructure. After the initialstrain sweep a recovery time of 300 s was given to allow forequilibration. Then a strain sweep from 0.0005 to 0.20 radians (in 30logarithmically spaced steps) at 20 Hz was performed and the dataanalyzed. The strain sweeps indicate a linear viscoelastic region overwhich the elastic modulus, G′, is constant with strain. At higherstrains the elastic modulus decreases with increasing strain. Theelastic modulus values in the plateau (constant) region are averaged andreported.

A similar protocol is used in determining and averaging the complexviscosity. Using the initial strain sweep and a 300 second delay timegives good reproducibility of strain sweep measurements. Reproducibilityon the same emulsion was shown to be ±7%. The measurements are performedin a darkened environment due to the fluorescent dyes incorporated inthe droplets. Solvent evaporation losses were negligible over the 20minute time period of a measurement; therefore a solvent trap was notused in these studies.

Methods for Assessing Hydrophobicity of the Colloidal Emulsifier

Various methods for determining the wetting characteristics of finelydivided solids have been reviewed by Kaya and Koishi (KONA, No. 6, pp.86-97, 1988). The preferred method for characterizing the wettability ofhydrophobized silica surfaces is infrared spectroscopy, as taught byFlinn et al. (Flinn, D. H., Guzonas, D. A. and R. H. Yoon, Colloids &Surfaces A: Physicochemical & Engineering Aspects, 87, pp. 163-167,1994).

Infrared spectra of particle emulsifiers were obtained by diffusereflectance infrared Fourier transform spectroscopy (DRIFTS) using aSpecac DRIFTS accessory in a Nicolet Magna 750 FT-IR spectrometer. Theamount of sample relative to the amount of ground KBr in the sample cellwas adjusted to obtain an absorbance of approximately 1 for the silicasamples and approximately 1.5 for the polyethylene powders. The internalreference for the silica samples was calculated by taking the peak areaof the CH₃ stretch at 2964 cm⁻¹ and dividing it by the peak area of anSiO band at 800 cm⁻¹ and then multiplying by 100. We define this as therelative hydrophobicity index.

Autoclaving

Sensors were autoclaved in loosely capped Pyrex jars containingcarbonate buffer (10.3 mM Na₂CO₃ buffer and 144 mM NaCl) using a liquidcycle at 121° C. for 1 hour. The temperature was verified using an Omega871 Digital Thermomoter with a Type K thermocouple (NiCr-NiAl) submergedin a Pyrex jar containing an equal volume of distilled water. Thesetests simulated sterilization conditions for a sensor product. A 3M“ATTEST” Steam Pack 1276 containing ATTEST 1262 BacillusStearothermophilis biological indicator may be used to validate theeffectiveness of the sterilization cycle.

Sensor Performance Testing

The robustness of sensors was evaluated by measuring sensor intensityand response to CO₂. Sensors were excited at approximately 465 nm andthe emission at 520 nm was measured using a xenon lamp in conjunctionwith an appropriate filter. Calibration slopes were determined bymeasuring the average intensities at 37° C. in buffer solutionstonometered with 2.8% and 8.4% CO₂. Measurements on individual sensorswere reported as the average of 25 data points. Intensities were alsomeasured at room temperature in air-sparged carbonate buffer (I_(air))and after exposure to 0.5% w/v ammonia solution in deionized water(I_(max)), the latter measurement of total dye concentration in thesensor permitting sensor intensities to be normalized.

The following examples are offered to aid in the understanding of thepresent invention and are not to be construed as limiting the scopethereof. Unless otherwise indicated, all parts and percentages are byweight except that CO₂ gas compositions are expressed by volume percentor in terms of partial pressure in mm Hg.

EXAMPLES Preparatory Example 1 Preparation of Stock Solutions

Various sensor emulsions were prepared by dispersing a water solubleaqueous phase (Stock A) and a hydrophobic emulsifier into a siliconecontinuous phase (Stock B).

To form an emulsion, 30 parts Stock A was mixed with 67 parts Stock Band 3 parts of a water insoluble emulsification enhancement agent. Thewater in oil emulsion was formed by homogenizing for 20 minutes at25,000 rpm on a Vertishear Cyclone/Tempest IQ homogenizer (availablefrom the Vertis Company) with macro blade assembly with cooling in anice water bath. Care was taken to keep the emulsion out of the light.

Stock A was prepared by dissolving a water soluble emulsifier andoptional humectants into a solution containing a fluorescent dye andoptional buffers and osmolytes. The mixture was then placed into a foilwrapped jar and shaken on a mechanical shaker overnight.

In the working examples, Stock A was composed of 2.7 mM HPTS(8-hydroxy-1,3,6-pyrene trisulfonic acid, trisodium salt—available fromEastman Kodak); 8.1 mM Na₂CO₃; and 144 mM NaCl. In a preferredembodiment, Stock A also contained 5% Starpol 530(hydroxypropyl-substituted polysaccharide—available from A.E. StaleyManufacturing Company, M_(w) ^(˜)500,000 to 600,000, M_(n) ^(˜)80,000 to90,000) and 5% Pluronic F108 (polyethylene oxide—polypropyleneoxide—polyethylene oxide block copolymer—available from BASF) added asthe humectant and water soluble emulsification enhancement agent,respectively.

Stock B was composed of Dow Corning (designated as “DC”) 7690 siliconewith 5% DC 7678 crosslinker. In a preferred embodiment, the waterinsoluble emulsification enhancement agent was CAB-O-SIL TS-530 fumedsilica, available from Cabot Corp., Billerica, Mass. (3% based on theweight of the entire sensor composition).

Three different mixing methods are exemplified. In mixing method #1,silica was added to Stock A and Stock B and homogenized together using aVertishear Cyclone Tempest IQ for 20 minutes at 25,000 rpm. In mixingmethod #2, the hydrophobic emulsifier was first added to Stock B anddispersed at 5000 rpm for 5 minutes on the Vertishear in an ice waterbath prior to adding to Stock A and homogenizing for 20 minutes at25,000 rpm. In still other examples where either no silica was used orwhere the silica was already incorporated into the commercial silicone,no additional filler was added before Stock A and B were homogenized at25,000 rpm for 20 minutes (mixing method #3). In the case of Example 8,Runs 3-5, the emulsion was sonicated in an ultrasonic bath duringhomogenization.

In order to form the cured sensors, a UV activated catalyst(cyclopentadienyl trimethyl platinum) was added in toluene and mixed byhand to give a final concentration of 0.02% in the final emulsion.Sensor emulsions were degassed at room temperature in a vacuum chamber,and then precision coated onto a polycarbonate web having a thickness of0.018 cm (available from Miles, Inc.) primed with an adhesionenhancement component derived from a mixture containing water, 1.25%colloidal silica particles, 0.11% aminopropyltriethoxysilane, 0.5%ammonium hydroxide and 0.03% of a surfactant sold by Rohm and Haas underthe trademark Triton X-100. Emulsions were precision coated using anotch bar coater or a coating apparatus sold by Hirano under thetrademark M-200. Emulsions were cured under UV light for 2 minutesfollowed by optional heat curing at 70° C. for 3 minutes. Coatingthickness was measured with a gage (Federal) to 0.0003 cm.

An opaque film precursor (Stock C) was overcoated onto the curedsilicone sensor. Stock C was composed of a dispersion of carbon black(Regal 99R—available from Cabot) in a poly(dimethyl)siloxane matrix(PLY-7501—available from NuSil Silicone Technology) having platinumcatalyst (Cat-50—available from NuSil Silicone Technology), andpolymerization inhibitor (XL 119—available from NuSil SiliconeTechnology). Stock C was precision coated as described above and curedat 70° C. for 2 minutes.

Alternatively, sensors were handcast directly onto an injection moldedpolycarbonate cassette at a thickness of 0.008 cm and UV cured. Sensorscassettes were hydrated in buffer containing 10.3 mM Na₂CO₃, and 144 mMNaCl buffer for 1 day at room temperature before testing.

Example 1 Presently Preferred Sensor Emulsion

This example illustrates the improvement of the invention relative tothe state of the art with respect to lot-to-lot consistency and emulsionstability. As shown in Table 1a, 9 g of Stock A (comprised of 5% StaleyStarpol 530 and 5% BASF Pluronic F108 in a 2.7 mM HPTS, 8.1 mM Na₂CO₃,and 144 mM NaCl solution) was added to 22 g Stock B (comprised ofsilicones DC 7690 and DC 7678) and 0.9 g (3%) water insolubleemulsification enhancement agent (in this example labeled “Filler”) andmixed by method #1, as described above.

TABLE 1a Composition of sensor Stock A Water soluble emulsificationMixing Run # Humectant enhancement agent Stock B Filler Method 1 5%Starpol 530 5% Pluronic F108 DC7690/7678 3% TS-530 1

The stability of uncured sensors emulsions is summarized in Table 1b, asdetermined by visual observations of inital droplet size homogeneity anddynamic rheological measurements over the course of 48 hours.

TABLE 1b Performance of Sensors Emulsions Visual Rheology observationson Initial droplet G′ (Pa) G′ (Pa) % Run # emulsion size (microns)(initial) (at 48 hrs) Change 1a Emulsion is 1-2 557 493 −11.5 viscous,not monodisperse pourable 1b Emulsion is 2-4 463 448 −3.2 viscous, notmonodisperse pourable 1c Emulsion is 2-4 371 333 −10.2 viscous, notmonodisperse pourable

Referring to Table 1b, the uncured sensor emulsions showed uniformlysmall and monodisperse droplet sizes in the range of 1 to 4 microns. Forall 3 replicates, the initial dynamic modulus was greater than 100 Pa,as would be expected by the visual appearance of the emulsions, whichappeared viscous and not pourable. The modulus changed much less than25% over 48 hours, indicating good aggregation stability for theseemulsions. No visual change in droplet size was observed after 2 days.In addition, the viscosity of Run 1 was optimal for precision coating.

The sensor emulsion was precision coated and UV cured, according to themethods previously described. The performance of coated and curedsensors are recorded in Table 1c for pre- and post-autoclaved sensors.

TABLE 1c Sensor performance Pre- Post Run Pre- autoclave Post autoclaveautoclave autoclave # I_(air) I_(air)/I_(max) I_(air) I_(air)/I_(max)slope (mm⁻¹) slope (mm⁻¹) 1 24,957 .97 21,358 .92 0.018 0.017

The intensities of the sensors in air-sparged buffer (I_(air)) weregreater than 20,000 counts, as would expected from a measurement of themaximum dye intensity (I_(max)) for a robust, stable sensor.Furthermore, the I_(air) intensities were reproducible and stable toautoclave (I_(air)/I_(max)>0.9), with no acidification of the sensoroccurring. Calibration slopes also were stable to autoclaving. Thestability of the sensor intensities and calibration slopes pre- andpost-autoclave is advantageous because consistent, reproducible andsterilization-insensitive calibration response is a requirement fortransparently calibrated sensors.

Example 2

Various sensor formulations using emulsification methods known in theart were prepared and compared to sensor formulations of the presentinvention. The formulations and performance characteristics aresummarized in Tables 2a, 2b, and 2c.

Run 1 illustrates that the presence of a humectant does not appear to beessential in promoting the stability of the gas sensing emulsion.Example 2, Run 1 was prepared identically to Example 1, Run 1 exceptwithout the humectant (Starpol 530) in Stock A. The emulsion shows adrop in G′ of only 20% after 48 hours, indicating comparable stabilityrelative to Example 1, Run 1 (with the humectant).

Comparative Run 2 corresponds to a gas sensor emulsion prepared withoutadded emulsifiers, as would be exemplified by AVL patent EP 0 105 870.The method used to prepare Comparative Run 2 differed from EP 0 105 870in that DC7690/7678 silicones were used rather than the Wacker Chemiesilicones SLM40060/40061 listed in the AVL patent since the Wackersilicones are not presently commercially available. Homogenizationconditions also differed. The emulsion was homogenized at 25,000 rpm for5 minutes instead of the 30 seconds with a “high speed mixer” (atunspecified speed) listed in the AVL patent. The Stock A was formulatedaccording to the AVL patent. The performance of the sensor emulsion isrecorded in Table 2b. The emulsion was significantly less stable thanthe emulsion of Example 1, Run 1. Unlike the Example 1, Run 1 emulsion,Comparative Run 2 was of very low viscosity and pourable, as evidence bythe low value for G′ (G′<<100 Pa). The emulsion droplet size ofComparative Run 2 was polydisperse at 2-8 microns. The emulsion ofComparative Run 2 was handcast into sensors, as shown in Table 2c.Compared to Example 1, Run 1, Comparative Run 2 showed significantlylower intensities with a small I_(air)/I_(max) and also exhibitedgreater variability in post-autoclave intensity.

Comparative Run 3 illustrates emulsion performance using a conventionallow HLB surfactant as an emulsifier. Such emulsifiers may sometimes bepredicted to yield stable water in oil systems. Pluronic L121 (HLB 0.5)was mixed into silicone Stock B at 5000 rpm for 5 minutes, prior tohomogenization with Stock A at 25,000 rpm for 20 minutes using mixingmethod #3. Like Comparative Run 2, the emulsion showed very lowviscosity and was visibly runny. The droplet size was polydisperse at2-20 μm. The dispersed aqueous phase rapidly coalesced, makingrheological measurements impossible. Stability was significantly poorerthan the emulsion of Example 1, Run 1 or Example 2, Run 1.

In Comparative Run 4, an emulsion was prepared with a hydrophobic,colloidal emulsifier (CAB-O-SIL TS-530 fumed silica), but with no watersoluble emulsification enhancement agent. Hydrophobic particles maysometimes act to stabilize water in oil emulsions. However, as shown inTable 2b, the resulting emulsion exhibited low viscosity and waspolydisperse (2-30 micron droplet size). Coalescence was visible,resulting in rapid phase separation. Rheology measurements could not bemade due to the poor stability of this emulsion. Because of pooremulsion stability, neither Comparative Runs 3 or 4 could be coated andcured to form emulsion sensors.

Comparative Run 5 illustrates yet another method of preparing anemulsion sensor composition using a high molecular weight, water solubleemulsifier added to the aqueous phase, as exemplified in U.S. Pat. No.5,219,527 and European Patent Application 0 597 566 A1 (Puritan-BennettCorporation). The emulsion was prepared by adding poly(vinylpyrrolidone)(PVP, 40,000 Da) to the aqueous phase Stock A (containing 10 mM HPTS and100 mM NaHCO₃) and emulsifying with Stock B (containing Petrarch PS784base silicone) at 25,000 rpm for 20 minutes. A portion of 10% PS123crosslinker was added and mixed by hand. As shown in Table 2b, theemulsion was not viscous and the droplet size was still polydisperse(2-24 μm). The initial elastic modulus was 13.6 Pa, as would be expectedfor a low viscosity, pourable emulsion. The modulus showed a 28.7% dropafter 48 hours, reflecting coalescence of the aqueous droplets, andgross phase separation was seen on standing. Upon precision coating,water droplets were observed to separate from the margins of the coatingprior to curing. Handcast sensors were tested, as shown in Table 2d. Theintensities were lower and more variable than in Example 1, Run 1.Intensities were reasonably stable to autoclave, however some intensityloss occurred.

Comparative Run 6 illustrates a sensor emulsion made with a highmolecular weight amphiphilic emulsifier (dextran; 500,000 Da) and asilicone (DC 7690) filled with hydrophobic colloidal particles, preparedas described in U.S. Pat. No. 4,867,919 and U.S. patent application Ser.No. 08/137,289. In contrast to earlier Comparative Runs 2 to 5, theemulsion is viscous, with an initial elastic modulus greater than 100 Paand a monodisperse droplet size (2-4 μm). Although the initial sensorintensities were comparable to those obtained in Example 1, Run 1, theintensity dropped precipitously by more than 70% post autoclave, to giveI_(air)/I_(max) approximately 0.27, indicating poor sterilizationstability of the gas sensing composition.

Comparative Run 7 illustrates the critical role of hydrophobic colloidalemulsifier, working in conjunction with the amphipathic, water solubleemulsification enhancement agent, on the stability of the sensoremulsion. Comparative Run 7 was prepared in an identical method toExample 1, Run 1 except that no hydrophobic colloidal silica was addedprior to homogenization. Contrary to Example 1, Run 1, which produced astable water in oil emulsion of uniform droplet size, Comparative Run 7yielded an unstable oil in water emulsion, having 20-40 micron oildroplets in a fluorescent aqueous continuous phase. The inversion of thewater-in-oil emulsion to form an oil-in-water emulsion was alsoreflected by the extremely high values of the elastic modulus. Becausethe emulsion inverted, it had no utility as a gas sensor and no sensorwas prepared.

TABLE 2a Composition of sensors Stock A Water soluble emulsificationenhancement Mixing Run # Humectant agent Stock B Filler Method 1 none 5%Pluronic DC7690/7678 3% TS-530 1 F108 C2 none none DC7690/7678 none 3 C35% Starpol 530 5% Pluronic DC7690/7678 none 3 L121 C4 5% Starpol 530none DC7690/7678 3% TS-530 1 C5¹ none 10% PVP PS784/PS123 none 3(40,000) C6 none 33% Dextran DC7690/7678 none 3 (500,000) C7 5% Starpol530 5% Pluronic DC7690/7678 none 1 F108 ¹Homogenization time was 5minutes at 25,000 rpm using the Vertishear.

TABLE 2b Performance of Sensor Emulsions Rheology Run Visualobservations on Initial droplet size G′ (Pa) G′ (Pa) % # emulsion(microns) (initial) (at 48 hrs) Change 1 Emulsion was well mixed, 2-12microns 743 594 −20.1 viscous and not pourable. Polydispersed C2Emulsion was well mixed, 2-4 microns with 2.3 2.5¹ 8.7 but low viscosity(pourable) occasional 6-8 micron droplets C3 Emulsion was well mixed,Polydispersed at 2-20 failed² failed failed but low viscosity and runnymicrons with coalescence C4 Water droplets seen on side Polydisperse at2-30 failed failed failed of jar and water phase microns and visiblyseparates when jar is coalescence seen. inverted. C5 Emulsion is notviscous (is Polydispersed at 2-24 13.6 12.3 −28.7 pourable) and watermicrons with many at separates at margins of 16 microns. coating. C6Emulsion was viscous (not 2-4 microns 104.6 32.6 −68.8 pourable).monodispersed C7 Emulsion was well mixed, Difficult to see 829 1,650+99.0% viscous and not pourable. individual fluorescent droplets. Largedark oil droplets of 20-40 microns seen. Inversion. ¹G′ values weretaken at 24 hours rather than 48 hours for this sample. The low G′values show that this sample is primarily viscous in nature. ²“Failed”denotes emulsion was too unstable to permit rheologicalcharacterization.

TABLE 2c Sensor performance Pre- autoclave Post autoclave Run # I_(air)I_(air)/I_(max) I_(air) I_(air)/I_(max) C2¹ 2506 .27 1384 .25 C5 12581.98 8985 .97 C6 23684 .90 6877 .27² ¹Comparative example 2 wasformulated without buffer as described in the AVL patent. ²These dataare a dextran emulsion prepared with PE 1055 silicones (as described inU.S. Pat. No. 4,867,919).

The preceding examples illustrate the improvements in stability andsensor performance obtained when a hydrophilic amphipathic emulsifier isused in conjunction with hydrophobic colloidal emulsifier in a water inoil emulsion based blood gas sensor. These improvements include shelflife stability of the emulsion for precision coating, autoclavestability, dry web stability, rapid rehydration, and transparentcalibration through coating uniformity and intensity stability.

While not wishing to be bound by any particular explanation ofmechanism, we believe that the these improvements result from thesynergistic interaction of the two emulsifiers at the water-oilinterface. As will be shown in the following examples, this synergisticeffect is not limited to one particular chemical class of materials.Other amphipathic water soluble emulsification enhancement agents usedwith other hydrophobic colloidal emulsifiers known to those skilled inthe art could be used in conjunction with a variety of suitablehydrophobic continuous phases to achieve similar performanceimprovements.

Example 3 Effect of Silicone Type on Sensor Emulsion Stability

This example illustrates that a wide range of hydrophobic continuousphases can be used in conjunction with this novel emulsification system.

Run 1 is the same formulation illustrated in Example 1, Run 1 and usesDow Corning 7690 base silicone and DC 7678 crosslinker. These emulsionsboth show good stability as judged by the monodisperse droplet size andsmall change in G′ over 2 days.

Similar results were obtained for a system using Nusil PLY7500 siliconeas Stock B (Run 2). The emulsion shows a monodisperse droplet size of2-4 microns and an initial G′ of 518 Pa, which changes by only −18%after 48 hours.

Run 3 was prepared as described in Run 2, but with hexamethyldisilazane(HMDZ) treated silica and mixing method #3. The emulsion had an initialG¹ of 159 Pa, which changes by only −3% after 48 hours.

Run 4 was prepared using Huls PE1055 silicone containing in situ treatedhydrophobic fumed silica as Stock B. As shown in Table 3b, the emulsionshows monodisperse droplets (2-4 microns), an initial G′ of 16,800 Pa,and a change in G′ after 48 hours of only 14 percent. Although thisillustrates an emulsion with excellent stability, it does not representa preferred embodiment, as the high elasticity of this emulsion makes itdifficult to precision coat with high uniformity.

All of the emulsions described in this example exhibited superiorstability, and illustrate that a variety of different silicone materialsmay be selected for use as the hydrophobic continuous phase. Thesesilicones represented a range of initial continuous phase viscosities of4,000 to 50,000 cS.

TABLE 3a Composition of sensors Stock A Water soluble emulsificationMixing Run # Humectant enhancement agent Stock B Filler Method 1 5%Starpol 530 5% Pluronic F108 DC7690/7678 3% TS-530 1 2 5% Starpol 530 5%Pluronic F108 Nusil PLY7500 3% TS-530 1 3 5% Starpol 530 5% PluronicF108 Nusil PLY7500 HMDZ silica 3 (treated in situ) 4 5% Starpol 530 5%Pluronic F108 Huls PE1055 silica (treated 3 in situ)

TABLE 3b Performance of Sensor Emulsions Rheology Run Visualobservations on G′ (Pa) G′ (Pa) % # emulsion Droplet size (microns)(initial) (48 hrs) Change 1 Emulsion is well mixed, 2-4 monodispersed371 333 −10.2 viscous and not pourable. 2 Emulsion is well mixed, 2-4monodispersed 518 423 −18.3 viscous and not pourable. 3 Emulsion is wellmixed, 2-12 polydispersed 159 154 −3.1 viscous and somewhat pourable. 4Emulsion is extremely 2-4 monodispersed 16,800 19,200 14.3 viscous andcan only be sheared for 10 min.

Example 4 Effect of Hydrophobicity of Hydrophobic Colloidal Emulsifieron Emulsion Stability

This example illustrates the importance of the relative hydrophobicityof the hydrophobic colloidal emulsifier on the emulsion stability.Relative hydrophobicity was determined using the IR method describedpreviously. Table 4a lists a variety of colloidal emulsifiers and theirrelative hydrophobicity.

Runs 1 to 5 illustrate the use of hydrophobically treated fumed silicawhere the relative hydrophobicity is greater than 2. All of thesesilicas were prepared by treating the base silica with a hydrophobicmolecule that replaces or covers hydrophilic hydroxyl groups. Like theemulsion of Example 1, Run 1, these emulsions all exhibit an initial G′greater than 100 Pa and a change of G′ after 48 hours of 25% or less. Asshown in Table 4c, good pre- and post-autoclave intensities were seenfor these formulations.

Comparative Run 6 illustrates the effect of using a hydrophiliccolloidal emulsifier such as CAB-O-SIL M5 in conjunction with anamphipathic water soluble emulsification enhancement agent (e.g.,Pluronic F108). CAB-O-SIL M5 fumed silica was mixed into siliconeaccording to Method #2 and homogenized with the standard aqueous phaseStock A. As shown in Table 4b, the emulsion showed inversion asevidenced by dark, nonfluorescent oil droplets with a diameter of 4-18microns in a fluorescent aqueous continuous phase. Sensors prepared fromthis emulsion showed low pre-autoclave intensity, as shown in Table 4c.Hydration of these sensors in carbonate buffer showed visible leachingof the fluorescent dye out of the emulsion, as would be expected for anoil in water emulsion in which the dye was not adequately immobilized insilicone. This resulted in a wide intensity variability, reflected inthe unusual I_(air)/I_(max) ratio of 6.78 (a mean of 3 replicates).

Comparative Runs 7 and 8 illustrate the use of a hydrophobically treatedfumed silica (CAB-O-SIL TS-610 and Degussa R972, respectively) where therelative hydrophobicity is less than 2. In both cases these silicas wereprepared by treating the base silica with dimethyldichlorosilane agents,a process that replaces surface hydrophilic hydroxyl groups withhydrophobic methyl groups thereby rendering the surface morehydrophobic. Both Comparative Runs 7 and 8 yielded water in oilemulsions of very low viscosity which rapidly phase separated, asdetected visually. No rheology or sensor performance measurements couldbe made due to this rapid phase separation of the dispersed aqueousphase.

TABLE 4a Composition of sensors Stock A Relative Run Water solubleMixing hydrophobicity # Humectant EEA Stock B Filler Method (IR) 1 5%Starpol 5% Pluronic DC7690/7678 3% Cabosil 1 9.67 530 F108 TS-530 2 5%Starpol 5% Pluronic DC7690/7678 3% Degussa 1 5.65 530 F108 R812 3 5%Starpol 5% Pluronic DC7690/7678 3% Degussa 1 6.93 530 F108 R812S 4 5%Starpol 5% Pluronic DC7690/7678 1.6% Cabosil 2 9.58 530 F108 TS-720 5 5%Starpol 5% Pluronic DC7690/7678 3% Degussa 1 11.1 530 F108 R202 C6 5%Starpol 5% Pluronic DC7690/7678 3% Cabosil 2 0 530 F108 M5 C7 5% Starpol5% Pluronic DC7690/7678 3% Cabosil 2 1.70 530 F108 TS610 C8 5% Starpol5% Pluronic DC7690/7678 3% Degussa 2 1.72 530 F108 R972

TABLE 4b Performance of Sensor Emulsions Rheology Run Visualobservations on G′ (Pa) G′ (Pa) % # emulsion Droplet size (microns)(initial) (48 hrs) Change 1 Emulsion is well mixed, Monodispersed at 1-2530 453 −14.5 viscous and not pourable. microns 2 Emulsion was wellmixed, Monodispersed at 2-4 922 748 −18.9 viscous and not pourable.microns 3 Emulsion was well mixed, Fluorescent droplets 537 452 −15.8viscous and not pourable. are <2 microns 4 Emulsion is well mixed andPolydispersed at 2-14 145 117 −19.3 pourable (not runny) microns 5Emulsion is well mixed, Polydispersed at 2-6 556 415 −25.4 viscous andnot pourable microns C6 Emulsion was well mixed, Individual dropletsfailed failed failed viscous and not pourable. were not seen. DarkInversion of emulsion oil droplets of 4-18 observed. Storage buffermicrons seen under appears cloudy and fluorescence. fluorescent. C7Emulsion has very low Phase separated failed failed failed viscosity(pourable, runny) and is visibly phase separated. C8 Emulsion has verylow Phase separated failed failed failed viscosity (pourable, runny) andis visibly phase separated

TABLE 4c Sensor performance Pre- autoclave Post autoclave Run # I_(air)I_(air)/I_(max) I_(air) I_(air)/I_(max) 1 24,957 .97 21,358 .92 2 24,795.96 21,288 .97 3 — — — — 4 20,716 .96 27,564 .92 5 — — — — C6 1,083 6.78— — C7 — — — — C8 — — — —

This example illustrates that the hydrophobic colloidal emulsifierpreferably has a relative hydrophobicity (as defined by FT-IR) greaterthan 2. This relative hydrophobicity can be obtained through a varietyof surface treatments and manufacturing processes, as evidenced by theresults from several chemically distinct fumed silicas obtained fromdifferent suppliers using different manufacturing processes.

Comparative Example 5 Effect of Particle Size of Hydrophobic ColloidalEmulsifier on Sensor Emulsion Stability

This example shows the effect of particle size of the hydrophobicemulsifier on the stability of the sensor emulsion. The stability of thesensor emulsion was examined using hydrophobic particle emulsifierswhere the radius of curvature was comparable to or greater than theemulsion droplets. Referring to Table 5a, particles composed ofhydrophobic polyethylene waxes (Allied Signal, Accumist B series) and apartially oxidized (less hydrophobic) polyethylene (Allied Signal,Accumist A) with primary particle size of 6 and 12 microns were used insensor emulsions. In Run 4, a hydrophobic clay (NL Chemicals, Inc.,Bentone SD-2) with a primary particle size of 20 microns was alsoevaluated. All of these systems immediately yielded visually unstableemulsions, indicating that the particles were unable to orient at thewater/silicone interface due to their high radius of curvature andsurface structure. This is in stark contrast to the emulsion of Example1, Run 1, which contains a colloidal hydrophobic fumed silica having aprimary particle size of approximately 50 nm.

TABLE 5a Composition of sensors Stock A Water soluble emulsificationFiller Run enhancement Size Mixing # Humectant agent Stock B Particle(microns) Method C1 5% Starpol 5% Pluronic DC7690/7678 3% Acumist 6 1530 F108 A6 C2 5% Starpol 5% Pluronic DC7690/7678 3% Acumist 6 2 530F108 B6 C3 5% Starpol 5% Pluronic DC7690/7678 3% Acumist 12 2 530 F108B12 C4 5% Starpol 5% Pluronic DC7690/7678 3% Bentone 20 2 530 F108 SD-2

TABLE 5b Performance of Sensor Emulsions Rheology Run Visualobservations on Initial droplet size G′ (Pa) G′ (Pa) % # emulsion(microns) (initial) (48 hrs) Change C1 Emulsion had a very lowPolydisperse droplets failed failed failed viscosity and was pourable.of 1-10 micron were It was visibly phase aggregated in separated with aclear oil nonfluorescent, clear layer on the top of the oil. emulsion,and a grainy, yellow sediment separates on standing. C2 Emulsion wasinitially well 2-14 micron poly- failed failed failed mixed and had alow dispersed and viscosity (pourable). It flocculated droplets visiblyphase separated and with fluorescent, phase became grainy. separatedregion. C3 Emulsion was viscous and Difficult to assign a failed failedfailed not pourable. Cured sensors fluorescent droplet werenon-fluorescent. size; dark oil droplets of 10-100 micron. Inversion ofemulsion. C4 Emulsion was tan in color Droplets are failed failed failedand grainy with clay polydisperse (2-40 particles separating on micron)standing.

Example 6 Effect of Water Soluble Emulsification Enhancement AgentStructure on Sensor Emulsion Stability

This example demonstrates the effect of water soluble emulsificationenhancement agent structure on the stability of the sensor emulsion forwater soluble emulsification enhancement agents with a variety ofstructures and molecular weights. These runs illustrate the preferredcharacteristics (water solubility and amphipathic nature) and mostpreferred characteristics (high molecular weight, i.e., greater than2,000 Da) of the water soluble emulsification enhancement agent.

Comparative Run 1 illustrates the use of a nonionic, low molecularweight, amphipathic, low HLB, water insoluble emulsifier (I.C.I. Span85) added to stock A and used in conjunction with a hydrophobiccolloidal emulsifier (CAB-O-SIL TS-530) in the silicone phase. Asillustrated in Table 6b, this emulsion inverts to a silicone in wateremulsion, yielding oil droplets of 10-20 microns in diameter dispersedin a fluorescent continuous phase. The effect of emulsion inversion onsensor performance, as illustrated in Table 6c, is to dramatically lowerthe sensor intensity post-autoclave. In addition, the resulting sensorintensities (pre- and post-autoclave) are not reproducible, indicatingthat the emulsification method is not sufficiently robust formanufacturing transparently calibrated sensors.

Comparative Run 2 illustrates the use of an ionic, water soluble,amphipathic, high HLB emulsifier (I.C.I. G3300) added to Stock A andused in conjunction with CAB-O-SIL TS-530 and Stock B. Upon mixing, theemulsion showed monodisperse droplets in the range 2-4 microns, butexhibited low viscosity and low elasticity (G′=83 Pa), leading toaggregation instability as reflected by a drop in G′ of 41% after 48hours.

Comparative Run 3 illustrates the use of a nonionic, water insoluble,non-amphipathic emulsifier (Dow Polyglycol P4000, a polypropylene oxide)added to Stock B, used in conjunction with a hydrophobic colloidalemulsifier (CAB-O-SIL TS-530). As shown in Table 6b, the emulsiongrossly phase separated immediately after preparation. Comparative Runs1 and 3 illustrate the effect of water solubility of the water solubleemulsification enhancement agent in promoting stability of the gassensor emulsion, and in yielding blood gas sensing compositions whichexhibit good intensity stability.

Comparative Run 4 and Runs 5-8 illustrate the use of nonionic, watersoluble, non-amphipathic emulsifiers comprised of polyethylene oxide ofvarying molecular weights. As shown in Comparative Run 4, a lowmolecular weight polyethylene oxide (200 Da) imparts no stability to theemulsion, with phase separation seen immediately after preparation.Increasing the molecular weight of the water soluble emulsificationenhancement agent from 3350-300,000 Da (Runs 5-8) imparts some stabilityto the initial emulsions, but does not appear to impart long termstability to the emulsion, as reflected by a decrease in G′ after 48hours of 41 to 56%. This is in contrast to the emulsion of Example 1,Run 1, which shows less than a 10% change in G′ after 48 hours. Morepreferred water soluble emulsification enhancement agents have anamphipathic character and are effective in stabilizing the emulsion.

The benefits of amphipathic character of the water solubleemulsification enhancement agent is further illustrated by Run 9, whichuses a nonionic, high molecular weight, water soluble non-amphipathicemulsifier (Dow Polyglycol P15-200) in Stock A in conjunction withCAB-O-SIL TS-530 and Stock B. The Dow P15-200, while not amphipathic, isa copolymer composed of polyethylene oxide and polypropylene oxide, andis thus chemically similar to the Pluronic F108 amphipathic watersoluble emulsification enhancement agent used in Example 1, Run 1. Run 9produced an emulsion with an initial droplet size which was polydisperseat 2-10 microns. The emulsion of Run 9 showed less stability than theemulsion of Ex. 1, Run 1, as reflected by a 47% drop in G′ after 48hours.

Yet another example which illustrates the importance of the amphipathiccharacter of the water soluble emulsification enhancement agent isprovided by Run 10. Run 10 makes use of a nonionic, high molecularweight, water soluble non-amphipathic copolymer (UCON 75H-90000)composed of random blocks of polyethylene oxide and polypropylene oxide.Although the emulsifier has a similar molecular weight and chemicalcomposition to the most preferred water soluble emulsificationenhancement agent illustrated in Ex. 1, Run 1, it is less effective instabilizing the emulsion, yielding an initially polydisperse dropletdistribution of 2-8 microns and a 64% drop in G′ after 48 hours.

Comparative Run 4 and Runs 5 to 10 clearly illustrate the importance ofthe amphipathic character of the water soluble emulsificationenhancement agent in enhancing long term emulsion stability. While notwishing to be bound by any particular theory as to the mechanism, webelieve that the amphipathic nature of the emulsifier allows the watersoluble emulsification enhancement agent to orient at the water/oilinterface and interact with the hydrophobic colloidal emulsifier whichis also oriented at that interface, thus leading to enhanced aggregationstability.

Run 11 illustrates the use of a nonionic, low molecular weight, watersoluble, amphipathic, high HLB emulsifier (I.C.I. Tween 20) added toStock A and used in conjunction with CAB-O-SIL TS-530. Although theemulsion initially showed monodisperse droplets in the range of 2-4microns, the emulsion exhibited a 34% drop in G′ after 48 hours. Inaddition, sensors fabricated from the emulsion of Run 11 delaminatedduring autoclaving. While not wishing to be bound by any particulartheory, we believe that the delamination may have resulted from themigration of this relatively mobile, low molecular weight emulsifierduring autoclaving. Although this is a suitable water solubleemulsification enhancement agent, it illustrates an additional benefitof using a higher molecular weight emulsification enhancement agent.

The following runs illustrate the more preferred approach of ourinvention, namely, the use of an amphipathic, hydrophilic water solubleemulsifier in conjunction with a water insoluble, hydrophobic colloidalparticle emulsifier. The preferred emulsions show improved stabilityrelative to emulsification methods known in the art. An additionaladvantage of our novel emulsification method is improved sensorintensity stability, particularly after autoclaving.

Run 12 illustrates a nonionic, intermediate molecular weight, watersoluble, amphipathic, PEO-PPO block copolymer emulsifier (HypermerB261). Although the resulting emulsion is initially polydisperse at 3-14microns, the initial G′ drops only 25% over 48 hours, indicatingacceptable stability. In addition, sensors prepared from this emulsionshow good intensity both before and after autoclaving as shown in Table6c.

Run 13 shows the use of a nonionic, low molecular weight, water soluble,amphipathic (intermediate HLB) emulsifier (Silwet L77). The resultingemulsion exhibited an initial monodisperse droplet size of 2-4 microns,and stability as reflected by a drop in G′ of only 6% after 48 hours. Asshown in Table 6c, sensor intensities were acceptable both pre- andpost-autoclave.

Run 14 is similar to the emulsion of Ex. 1, Run 1. This emulsionexhibited an initial monodisperse droplet size of 1-2 microns, andstability as reflected by a drop in G′ of only 11% after 48 hours.

Run 15 illustrates a nonionic, high molecular weight, water soluble,amphipathic, BAB block copolymer emulsifier (PLURONIC-R - 25R8). Theresulting emulsion is initially polydisperse at 2-8 microns, and theinitial G′ drops only 7% over 48 hours, indicating good stability.

TABLE 6a Composition of sensors Stock A Water soluble emulsification Runenhancement agent # Humectant Type HLB Mol. wt. Stock B Filler C1 5%Starpol 530 5% Span 85¹ 1.8 1,100 DC 7690/7678 3% TS-530 C2 5% Starpol530 5% G-3300² 11.7 — DC 7690/7678 3% TS-530 C3 5% Starpol 530 5% P4000³— 4,000 DC 7690/7678 3% TS-530 C4 none 5% E-200⁴ — 200 DC 7690 3% TS-5305 5% Starpol 530 5% Carbo- — 3,000- DC 7690 3% TS-530 wax 3350⁵ 3,600 65% Starpol 530 5% Carbo- — 7,000- DC 7690 3% TS-530 wax 8000⁶ 9,000 7 5%Starpol 530 5% Carbo- — 15,000- DC 7690 3% TS-530 wax 20M⁷ 20,000 8 none5% PEO — 300,000 DC 7690 3% TS-530 N750⁸ 9 5% Starpol 530 5% P15-200⁹ —2,600 DC 7690/7678 3% TS-530 10 5% Starpol 530 5% UCON — 15,000 DC7690/7678 3% TS-530 75-H-90000¹⁰ 11 5% Starpol 530 5% Tween 16.7 1,226DC 7690/7678 3% TS-530 20¹¹ 12 5% Starpol 530 5% Hypermer 7-9 1,800 DC7690/7678 3% TS-530 B261¹² 13 5% Starpol 530 1% Silwet 5-8 600 DC7690/7678 3% TS-530 L77¹³ 14 5% Starpol 530 5% Pluronic 27 14,600 DC7690/7678 3% TS-530 F108¹⁴ 15 5% Starpol 530 5% Pluronic- 12 8,550 DC7690/7678 3% TS-530 R 25R8¹⁵ ¹“Span 85” is a sorbitan trioleateavailable from ICI Specialty Chemicals. ²“G-3300” is an ionic alkyl arylsulfonate available from ICI Specialty Chemicals. ³“P4000” is apolypropylene oxide available from Dow Chemical Corp. ⁴“E-200” is apolypropylene oxide available from Dow Chemical Corp. ⁵“Carbowax 3350”is a polyethylene oxide available from Union Carbide Corp. ⁶“Carbowax8000” is a polyethylene oxide available from Union Carbide Corp.⁷“Carbowax 20M” is a polyethylene oxide available from Union CarbideCorp. ⁸“Polyox N750” is a polyethylene oxide available from UnionCarbide Corp. ⁹“P15-200” is a random copolymer of polypropylene oxideand polyethylene oxide available from Dow Chemical Corp. ¹⁰“UCON75-H-90000” is a random copolymer of polypropylene oxide andpolyethylene oxide available from Union Carbide. ¹¹“Tween 20” is apolyoxyethylene (20) sorbitan monolaurate available from ICI SpecialtyChemicals. ¹²“Hypermer B261” is an ABA nonionic block copolymer withpolyhydroxy fatty acid as the hydrophobe and polyethylene glycol as thehydrophile, available from ICI Specialty Chemicals. ¹³“Silwet L77” is apolyalkylene oxide modified polydimethylsiloxane (block copolymer)available from OSi Specialities. ¹⁴“Pluronic F108” is apolyethyleneoxide-polypropylene oxide polyethylene oxide blockcopolymer, available from BASF. ¹⁵“Pluronic 25R8” is a polypropyleneoxide-polyethylene oxide polypropylene oxide block copolymer, availablefrom BASF.

TABLE 6b Performance of Sensor Emulsions Rheology Run Visualobservations on Initial droplet size G′ (Pa) G′ (Pa) % # emulsion(microns) (initial) (48 hrs) Change C1¹ Emulsion is well mixed andFluorescent held with failed failed failed viscous (not pourable). darkoil droplets (10- 20 microns). Emulsion inverted. C2 Emulsion is wellmixed and 2-4 monodispersed 83 49 −41.0 low viscosity (runny). C3Emulsion was phase separated, failed failed failed failed low viscosityand pourable with fluorescent sediment. C4 Unstable emulsion; phasefailed failed failed failed separation. 5 Emulsion is well mixed, 2-10microns, 487 248 −49.1 viscous and not pourable. polydispersed Clear oillayer seen on side of jar (phase separation). 6 Emulsion is well mixed,2-14 microns, 473 210 −55.6 viscous and not pourable. polydispersed 7Emulsion is well mixed, 2-4 microns, 545 301 −44.8 viscous and notpourable. monodispersed 8 Emulsion is well mixed, 2-4 microns, 405 240−40.7 viscous and not pourable. monodispersed 9 Emulsion is well mixed,2-10 microns, 452 239 −47.1 viscous and somewhat polydispersed pourable.10 Emulsion is well mixed, 2-8 microns, 458 163 −64.4 viscous, and notpourable. polydispersed 11 Emulsion is well mixed, 2-4 microns, 242 159−34.3 viscous and not pourable. monodispersed 12 Emulsion is well mixed,3-14 microns, 316 236 −25.3 viscous and somewhat polydispersed pourable.Color is darker due to amber colored Hypermer. 13 Emulsion is wellmixed, 2-4 microns, 137 129 −5.8 viscous and somewhat monodispersedpourable. 14 Emulsion is well mixed, 1-2 microns, 557 493 −11.5 viscousand not pourable. monodispersed 15 Emulsion is well mixed, 2-8 microns,408 378 −7.3 viscous and not pourable. polydispersed ¹This run was mixedusing Mixing Method #2; all other runs were mixed using Mixing Method#1.

TABLE 6c Sensor performance Pre- autoclave Post autoclave Run # I_(air)I_(air)/I_(max) I_(air) I_(air)/I_(max) C1 23,245 1.02 5828 1.03 C2 — —— — C3 — — — — C4 — — — — 5 — — — — 6 — — — — 7 — — — — 8 — — — — 9 — —— — 10 — — — — 11 28,732 .98 delaminated — 12 28,447 1.13 24,743 1.01 1324,762 .94 21,428 .94 14 — — — — 15 — — — —

Example 7 Effect of the HLB of the Water Soluble EmulsificationEnhancement Agent on Sensor Emulsion Stability

This example illustrates the wide HLB range of the amphipathic, watersoluble emulsification enhancement agent which confers stability to theemulsion. The emulsifiers shown in Table 7a are all nonionic, watersoluble, amphipathic ABA block copolymers of the type PEO-PPO-PEO. Theseemulsifiers cover a range of molecular weights (1,900 to 4,400 Da) andHLB's (1-19), but have been chosen such that the molecular weight of thePEO block is approximately constant at 400-500 Da.

Comparative Run 1 illustrates the use of a classical low HLB, waterinsoluble emulsifier, namely Pluronic L121. Comparative Run 1 yielded aninverted emulsion of 100 micron oil droplets. The emulsion exhibited lowviscosity and was visibly phase separated.

Runs 2 and 3 illustrate the use of higher HLB emulsifiers, namelyPluronic L44 and L35, respectively. Runs 2 and 3, taken in combinationwith Ex. 1, Run 1, and Example 6, Runs 11-15, illustrate that goodemulsion stability is obtained over a wide range of HLB, provided thatthe HLB is at least 5. This is an unexpected result based upon theconventional rule (Bancroft's rule), which predicts that low HLBemulsifiers (e.g., HLB 1-4, such as L121) are most effective at formingand stabilizing a water in oil emulsion. For a discussion of Bancroft'srule see, Shaw, Introduction to Colloid & Surface Science, 3rd Ed.,Butterworths, London, 1983, p. 237.

TABLE 7a Composition of sensors Stock A Water soluble emulsificationenhancement agent Run Mol. wt. of # Humectant Type HLB PEO block Stock BFiller C1¹ 5% Starpol 530 5% L121 1 400 DC 7690 3% TS-530 2 5% Starpol530 5% L44 16 500 DC 7690 3% TS-530 3 5% Starpol 530 5% L35 19 500 DC7690 3% TS-530 ¹Runs 1-3 were mixed using Mixing Method #2.

TABLE 7b Performance of Sensor Emulsions Rheology Run Visualobservations on G′ (Pa) G′ (Pa) % # emulsion Droplet size (microns)(initial) (48 hrs) Change C1 Emulsion has a very low 2-20 polydispersedfailed failed failed viscosity, pourable (runny), with dark oil dropletsgrainy and visibly phase (100 micron). separated. Emulsion inverted. 2Emulsion is well mixed, 2-8 polydispersed 265 210 −20.8 viscous andsomewhat pourable. 3 Emulsion is well mixed, 2-10 polydispersed 306 259−15.4 viscous and somewhat pourable.

Example 8 Effect of the Humectant on Sensor Emulsion Stability

This example illustrates the wide range of humectants which can beincorporated into the emulsions in order to obtain additional desirablesensor performance characteristics. These additional characteristics mayinclude improved dry web intensity stability, rapid rehydration of thesensor, and response to dry gas (such as CO₂ in the air). In the mostpreferred case, the humectants are added to the stabilized emulsions asdescribed. As is most evident with the emulsion containing glycerol,trehalose, xanthan gum or Starpol humectants, the absence of thehydrophilic amphipathic emulsifier provides grossly phase separatedemulsions immediately after mixing.

In contrast, as shown in Tables 8a and 8b, humectants with weightaverage molecular weights of 92 Da to 1,000,000 Da can be incorporatedinto a stable sensor emulsion when a hydrophilic emulsifier is used inconjunction with a water insoluble, colloidal particle (such as silica).This illustrates an additional useful feature of our novel emulsifiersystem, namely the ability to incorporate a wide range of humectantswith varying molecular weights and structures into the same sensorformulation, thereby permitting the sensor performance to be formulatedfor specific applications. This has also been illustrated in Example 2,and further demonstrates that the choice of humectant is important toachieving autoclavability of the sensor, as measured by the stability ofsensor intensities and slopes.

The utility of this approach is further illustrated in Tables 8c and 8d.As shown in these Tables, the choice of humectant is important toachieving manufacturability of precision coated sensors by stabilizingthe sensor intensity in a dry coated web. This is important inmanufacturing because it extends the shelf life of the web and thereforeallows a longer time for the sensor web to be converted into product.Superior intensity stability was demonstrated with sensor emulsionscontaining xanthan gum (Ex. 8, Run 3) and Starpol (Ex. 8, Run 1)humectants.

In addition to autoclavability and dry web stability, appropriate choiceof humectants also gives rise to a new sensor that responds to CO₂ fromthe air (as opposed to tonometered blood or buffer solutions), as shownin Table 8d for Ex. 8 Run 5. The response time is extremely fast, whichis a further advantage of this system.

This ability to sense dry gas was not conferred by ethylene oxide,polyethylene oxide, or polysaccharide based humectants (includingStarpol, xanthan gum, trehalose, or dextran). It is, however, conferredby glycerol present at 2-99% of the dispersed phase. A dry gas CO₂sensor has utility as a monitor for breath monitoring, monitoring thecorrect placement of endotracheal tubes, atmospheric industrialmonitoring, or for any application for which the sensor is notmaintained in equilibrium with water.

TABLE 8a Composition of sensors Stock A Run Water soluble emulsification# Humectant enhancement agent Stock B Filler 1¹ 5% Starpol 560 5%Pluronic F108 DC 7690 3% TS-530 (M_(w) ^(˜) 915,000, M_(n) ^(˜) 135,000Da) 2 30% Dextran 5% Pluronic F108 DC 7690 3% TS-530 (500,000 Da) 3 1%xanthan gum 4% Pluronic 10R8 DC 7690 3% TS-530 (1,000,000 Da) 4 11.4%Trehalose 5% Pluronic F108 DC 7690 3% TS-530 (378 Da) 5 30% glycerol (92Da) 5% Pluronic F108 DC 7690 3% TS-530 6 5% PEO (300,000 Da) none DC7690 3% TS-530 ¹Runs 1-6 were mixed using Mixing Method #1.

TABLE 8b Performance of Sensor Emulsions Rheology Run Visualobservations on G′ (Pa) G′ (Pa) % # emulsion Droplet size (microns)(initial) (48 hrs) Change 1 Emulsion is well mixed, 2-4 monodispersed230 259 12.6 viscous and not pourable. 2 Emulsion is well mixed, 2-4monodispersed 458 439 −4.51 viscous and not pourable. 3 Emulsion is wellmixed and 2-4 monodispersed 339 348 2.65 viscous. 4 Emulsion is wellmixed and 2-6 monodispersed 371 275 −25.0 viscous. 5 Emulsion is wellmixed and 2-6 monodispersed 340 331 2.65 viscous. 6 Emulsion is wellmixed and 2-4 monodispersed 405 240 −40.7% viscous

TABLE 8c Days at room Run # temperature % Intensity loss 1 0 0 19 11 39−7 3 0 0 19 −4 39 −2 6 0 0 19 −20 39 −27

TABLE 8d pCO₂ Time (minutes) Intensity (unreferenced) Air 0 to 3.3 295to 2.8% 3.4 237 3.5 198 3.6 180 3.7 173 3.8 to 6.0 172 to 8.4% 6.2 1666.3 166 6.4 166 6.5 166 6.6 123 6.7 to 9.0 122 to Air 9.1 167 9.2 1979.3 221 9.4 240 9.5 256 9.6 267 9.7 277 9.8 283 9.9 290 10.0 293 10.1 to12.0 302 to 2.8% 12.2 215 12.3 190 12.4 179 12.5 174 12.6 to 15.3 173 to8.4% 15.4 150 15.5 150 15.6 126 15.7 123 15.8 to 18.1 123 to Air 18.2149 18.3 182 18.4 210 18.5 232 18.6 251 18.7 265 18.8 274 18.9 286 19.0290 19.5 to 21.0 304

Example 9 Preparation of Sensors

A sensor emulsion was prepared by dispersing a first phase (Stock A)into a second phase comprising a hydrophobic silicone. To form theemulsion, 50 parts Stock A was mixed with 100 parts Dow Corning 7690vinyl silicone polymer and 3 parts of a water insoluble emulsificationenhancement agent (Cabosil TS-720 silica).

Stock A was prepared by mixing the following components: 50 partsglycerol; 40 parts polyethylene glycol-600; 10 parts of a 1M Na₂CO₃solution in water (to achieve a 100 mM Na₂CO₃ level); 0.21 parts HPTS(to achieve a 4 mM level); and 0.24 parts glycerophosphoric aciddisodium salt (to achieve an 8 mM level). The emulsion was formed byhomogenizing for 30 seconds with a Tissue-Tearor mixer operating atmaximum RPM.

A 0.5 gram aliquot of the resulting emulsion was removed and 20microliters of a UV-activated hydrosilation catalyst was added. One drop(^(˜)0.03 gm) of Dow Corning 7678 silyl hydride crosslinker was thenadded and stirred in. The mixture was briefly degassed under vacuum,then cast into the wells of a standard sensing cartridge. The assemblywas held under a UV sunlamp for three minutes to facilitate curing ofthe emulsion.

The sensing cartridge was then placed on a monitoring fixture andintensity under conditions of exposure to ambient room air was thenmeasured as 1070 counts. The sensor was then placed in a dry 60° C. ovenovernight and then held in ambient air for two days, after which theambient air intensity was measured as 1066 counts. The sensor was againheld in ambient air for 7 and for 30 additional days. The intensity was1021 and 1008, respectively. The sensor thus displayed a stable andeffective signal when stored under exposure to ambient air.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein.

What is claimed is:
 1. A gas sensing composition, comprising: adispersed first phase comprising droplets which are substantiallysmaller in at least one dimension than the thickness of the sensingcomposition, wherein the first phase contains at least one substantiallywater soluble emulsification enhancement agent and at least one watersoluble indicator component effective to provide a signal in response tothe concentration of a gas in a medium to which the sensing compositionis exposed; and a hydrophobic second phase which is permeable to theanalyte and impermeable to ionized hydrogen, wherein the second phasecontains at least one substantially water insoluble emulsificationenhancement agent; wherein the water soluble emulsification enhancementagent comprises a nonionic, amphipathic copolymer containing bothhydrophilic and hydrophobic moieties.
 2. A gas sensing compositionaccording to claim 1, wherein the hydrophilic moiety is polyethyleneoxide and the hydrophobic moiety is polypropylene oxide.
 3. A gassensing composition according to claim 1, wherein the water solubleemulsification enhancement agent is an ABA block copolymer, wherein theA block is a polyethylene oxide molecule and the B block ispolypropylene oxide molecule.
 4. A gas sensing composition according toclaim 1, wherein the water soluble emulsification enhancement agent is aBAB block copolymer, wherein the A block is a polyethylene oxidemolecule and the B block is polypropylene oxide molecule.
 5. A gassensing composition according to claim 1, wherein the water solubleemulsification enhancement agent has an HLB of at least
 5. 6. A gassensing composition according to claim 1, wherein the water solubleemulsification enhancement agent has an HLB of at least
 10. 7. A gassensing composition according to claim 1, wherein the water solubleemulsification enhancement agent has a weight average molecular weightbetween 100 and 50,000.
 8. A gas sensing composition according to claim1, wherein the water soluble emulsification enhancement agent comprisesa nonionic, amphipathic copolymer having a weight average molecularweight between 500 and 20,000.
 9. A gas sensing composition according toclaim 1, wherein the water soluble emulsification enhancement agentcomprises a nonionic, amphipathic copolymer and is present in aconcentration of between 0.01 and 5 weight % in the sensing composition.10. A gas sensing composition according to claim 1, wherein the secondphase comprises a carbon dioxide permeable polymeric material.
 11. A gassensing composition according to claim 1, wherein the second phase is asilicone material.
 12. A gas sensing composition according to claim 1,wherein the droplets have an average size less than 5 microns.
 13. A gassensing composition according to claim 1, wherein the dispersed firstphase further includes a buffer and an osmoregulatory agent.
 14. A gassensing composition according to claim 13, wherein the buffer isselected from the group consisting of bicarbonate or phosphate ion basedbuffer solution.
 15. A gas sensing composition according to claim 13,wherein the dispersed first phase has a pH from about 5 to
 14. 16. A gassensing composition according to claim 1, wherein the indicatorcomponent is a pH sensitive dye.
 17. A gas sensing composition of claim1, wherein the indicator component is selected from the group consistingof: hydroxypyrene trisulfonic acid, and salts of hydroxypyrenetrisulfonic acid.
 18. A gas sensing composition according to claim 1,wherein the volume of the aqueous phase occupies 10 to 60% of thesensing composition.
 19. A gas sensing composition according to claim 1,wherein the hydrophobic particles are present in a concentration ofbetween 0.1 and 20 weight % in the sensing composition.
 20. A gassensing composition according to claim 1, wherein the initial elasticmodulus of the uncured emulsion is greater than 100 Pa and theequilibrium elastic modulus at 48 hours is greater than 100 Pa.
 21. Agas sensing composition according to claim 1, wherein the initialelastic modulus of the uncured emulsion is greater than 200 Pa and theequilibrium elastic modulus at 48 hours is greater than 200 Pa.
 22. Agas sensing composition according to claim 1, wherein the waterinsoluble emulsification enhancement agent comprises a plurality ofdispersed hydrophobic particles.
 23. A gas sensing composition accordingto claim 22, wherein the hydrophobic particles have a mean volumeparticle size less than 5 microns.
 24. A gas sensing compositionaccording to claim 22, wherein the hydrophobic particles comprisesurface treated colloidal silica.
 25. A gas sensing compositionaccording to claim 22, wherein the hydrophobic particles are chemicallybonded to the hydrophobic second phase.
 26. A gas sensing compositionaccording to claim 22, wherein the second phase comprisespolydimethylsiloxane.
 27. A gas sensing composition according to claim22, wherein the initial elastic modulus of the uncured emulsion isgreater than 300 Pa and the equilibrium elastic modulus at 48 hours isgreater than 300 Pa.
 28. A sensor for measuring the concentration of ananalyte in a medium comprising: a sensing element comprising a sensingcomposition according to claim 22; an excitation assembly positioned andadapted to provide an excitation signal to the sensing element; adetector assembly positioned and adapted to detect an emitted signalfrom the sensing element, the sensing element being capable of providingthe emitted signal in response to being exposed to the excitationsignal; and a processor assembly positioned and adapted to analyze theemitted signal in determining the concentration of the analyte in themedium.
 29. A sensor for measuring the concentration of an analyte in amedium comprising: a sensing element comprising a sensing compositionaccording to claim 1; an excitation assembly positioned and adapted toprovide an excitation signal to the sensing element; a detector assemblypositioned and adapted to detect an emitted signal from the sensingelement, the sensing element being capable of providing the emittedsignal in response to being exposed to the excitation signal; and aprocessor assembly positioned and adapted to analyze the emitted signalin determining the concentration of the analyte in the medium.